The doorbell rang just after lunch, while they were cleaning the dishes. "Can you get that?" asked his mum, rinsing the soap from a plate.

Pleased to have been entrusted with such an important task, Jack half-ran to the door and then paused for a moment to pull himself up to his full height of almost four feet, to make an impression on whoever was at the door.

The man on the doorstep looked disappointingly unimpressed. He was tall, grey-haired, dressed mostly in black, and he was holding a pint of milk in one hand.

"Are you a magician?" asked Jack.

"Do I look like a magician?" asked the man, who sounded Scottish.

Jack nodded. "Yes."

"That just proves that you don't know many magicians," said the man.

Jack was about to argue this point when his mum appeared at his side, drying her hands on a tea-towel. The magician held out the carton in his hand. "You said to bring a pint of milk."

"That was four years ago," said Mum.

The man frowned. "Are you sure?"

"Pretty sure, yes." She took the milk anyway. "Thanks for remembering, I suppose." She held the door open for him. "Come in, you idiot."

Jack followed them into the living-room, vaguely hoping that the magician would put on some sort of show for them. Maybe they went round houses, like window-cleaners and people with leaflets about Jesus. He had to admit that there was a lot he didn't know about magicians.

The man took the armchair nearest the television. "How's the cough, Jack?"

"Four years," said Mum, sounding a bit annoyed. "I don't think he even remembers who you are."

"Of course he does! Who could forget me?" The man looked at Jack expectantly.

Mum sighed. "Doctor, he was tiny last time you were here."

"So?"

She sat down on the sofa. "So small children have a lot on their minds," she said. "Jack, this is the Doctor, he's a very good friend of mine."

"Clara, he knows who I am. Don't you?"

Jack shook his head, slightly mystified.

The Doctor scowled. "You don't have to lie to make her feel better."

Jack decided that he didn't like this new adult very much. He hopped up onto the sofa next to Mum and stared openly at the newcomer.

The Doctor scratched his head. "Seriously, four years?"

Mum nodded. "I was fairly sure you'd come back eventually. I thought you might have been off having adventures with an impressionable young woman. A different impressionable young woman."

"Oh, I hardly ever do that," said the Doctor. He looked round the room. "You've still got those ugly curtains."

"I missed you too," said Mum. "It's okay, you don't have to show your feelings in front of other people."

"I didn't think it'd been that long," said the Doctor, sounding genuinely apologetic. "It's funny how time flies when you're not paying attention to it. Anyway," he continued, "here we all are, safe and sound. The old gang back together."

Jack tugged on his mother's sleeve. "Mum, is he going to do magic tricks for us?"

"I'm not a magician," said the Doctor. He spread his hands. "Are these the hands of a magician?"

Jack nodded. "Yes."

The Doctor sat back with a sigh. "Kids these days." He pointed at Jack. "And what are you? Are you an astronaut?"

"I'm six years old," said Jack, quite reasonably.

"That's no excuse! I was out exploring the universe before I was -"

"Doctor," said Mum, in a warning tone. "We don't tell Jack stories about things that aren't real."

The adults looked at each other with an odd intensity. Jack looked between them, but he couldn't work out what they weren't saying.

Finally the Doctor nodded. "You're right, I'll stick to the truth." He looked at Jack. "Your mother is scrupulously honest about these things. It makes her a bit boring, but don't tell her I said so. Anyway," he said more generally, "this is just a quick visit to see how everyone is. I've got an appointent with... um... with a dentist." He glanced at Mum. "Dentists are real, aren't they?"

Jack wondered if this would all make sense when he was older.

The Doctor came back three weeks later, without milk this time but with a present for Jack that turned out to be quite a good toy spaceship that made noises and had flashing lights on it. Jack ran around the flat with his new acquisition while Mum and the Doctor made dinner. Occasionally he overheard bits of conversation, but they didn't seem interesting enough to halt exploration of the home-shaped universe.

"Was he always that tall?" asked the Doctor.

"They grow," said Mum. "They start out small and they keep getting bigger until they stop growing."

"I know that, Clara, I've read about it in books."

"He's a bit small for his age, actually. Not a worrying amount," she added.

"Don't worry, I was tiny child myself, and look at me now."

"Are you taller than you used to be?" she asked, which seemed like an odd thing to ask a grown-up.

"I don't think so, but I didn't measure myself last time. I should start doing that. I could keep a graph of my own height." His voice went quieter. "Probably shouldn't talk about it with Jack around."

"You're right, I wasn't thinking." She sighed. "It's a bit of a minefield, really."

"Don't chop the onions like that, they'll go for your eyes. Here, I'll get those and you look after the chicken."

"You're being very domestic," said Mum.

"I know, frankly it's terrifying me. I could pass out from fear at any moment"

"Daleks and vegetables," she said, "that's an interesting set of weaknesses."

"What's Daleks?" asked Jack on his next run through the kitchen.

"I told you he was eavesdropping," said the Doctor. "Look at his ears, they're like satellite dishes. Daleks are a fruit," he added, "with seeds on the outside and a shiny skin."

Jack accepted this information with a nod. He landed his spaceship on the counter-top next to Mum and looked up at her. "Can I go outside to play?"

"As long as you don't wander off, your dinner will be ready soon." She leaned down to kiss his cheek and then waved him out of the room. "I don't know where he gets all that energy," she said as he ran out of the room.

"I usually blame the parents," said the Doctor, but the rest was cut off as Jack closed the front door.

The next morning Jack wandered into the kitchen looking for his spaceship and found the Doctor sitting at the table eating a bowl of cornflakes. This was unusual, since adults usually waited until at least afternoon to visit each other. Jack climbed onto a chair opposite the Doctor and stared at him for some time before he got any sort of response.

"There's still some left," said the Doctor, sliding the box of cereal across the table. "I didn't touch the Coco Pops, which I assumed were yours."

Jack was pleased that the Doctor seemed to understand the boundaries of hospitality. "Where do you live?" he asked, as a polite conversation-starter and not sheer nosiness.

"In a box," said the Doctor between spoonfuls of cornflakes.

"Are you homeless?"

He shook his head. "Not that sort of box. It's more of a caravan. A really good caravan."

"I've been in a caravan," said Jack. "We lived in it for a week when we went on holiday. It wasn't very good."

"This is much better than that caravan," said the Doctor.

"How do you know?"

"Because I just do." He finished his breakfast and pushed the bowl into the centre of the table. "Do you have a job?"

"I go to school," said Jack, "but we're off until August."

"Do you like school?"

"Of course he does," said Mum, appearing at the kitchen door in her dressing-gown. "He's the best reader in his class."

The Doctor nodded. "Naturally, he has..." he paused, seemingly uncertain, "...he has your brains." A look passed between the adults that Jack couldn't work out.

"Can you take him to the park?" asked his mum. "I've got an appointment at the hairdresser's."

"I didn't want to say anything," said the Doctor, "but you should probably go before it gets worse."

"Thanks."

"You're very welcome."

"I spy with my x-ray eye... something beginning with... S."

"Swings," said Jack.

"Oh, very good," said the Doctor. "You're far too good at this. Your turn."

They were sitting on the swings in the little park next to the police station. The Doctor had a bag of chips, and was being quite good about remembering to share.

"Can I ask a question?" asked Jack.

"Is it personal?"

"It might be," said Jack, uncertain.

"Well, you can always ask, but I might not answer it."

Jack took a moment to compose his thoughts, and then said "Are you my dad?"

"Don't be daft," said the Doctor. "That's the stupidest thing you've ever said."

Jack considered this. "Okay," he said. "Are you going to be my dad?"

"What?"

"My mum likes you," said Jack, starting a list on his fingers, "and you need somewhere to live. You're quite good for a grown-up, so I wouldn't mind if you wanted to be my dad."

The Doctor looked at him a bit strangely. "A lot of people don't like me," he said. "If I was your dad they'd probably try to melt you, just to annoy me. You don't want to be melted, do you?"

Jack gave the idea some thought. "Not really."

"I didn't think you would." The Doctor shook his head. "You don't want a dad, that's just an extra person to make you brush your teeth and comb your hair. Do you want another chip?"

Jack took a chip and chewed on it slowly as he thought things over.

"Don't worry about it," said the Doctor, "life's complicated enough as it is."

The next time Jack saw the Doctor was just after school started back. He came home from a hard day of spelling and colouring things in, dropped his bag in the hallway, and followed the sound of voices into the living-room. The Doctor was sitting in the armchair and Mum was putting antiseptic on a cut just above his left eye.

"What happened?" asked Jack, not terribly worried but still quite interested.

"I fell off a roundabout," said the Doctor, trying to duck away from Mum's attentions. "She's making a big deal out of it, though."

"Does it hurt?"

"It didn't hurt until someone put antiseptic on it, then it started to sting."

Mum shook her head. "You're not fooling anyone, Doctor."

"I'm fine," he said, waving her away. "Look, there's Jack, why don't you go bother him?"

"You have to stop the germs getting in," said Jack, in case the Doctor didn't know about medicine.

"I'm immune to germs," was the rather petulant reply.

Mum put the first aid kit away. "One of these days -"

"One of these days you'll stop worrying," said the Doctor.

"I won't, you know."

The Doctor stood. "It was nice seeing you, sorry to rush off, things to do." He ruffled Jack's hair as he passed him. "You look after your mother."

Mum sat down as the front door closed behind the Doctor. "He's impossible, he really is."

Jack wasn't really sure what to say to that, so he nodded and tried to look grown up.

He was woken in the middle of the night by a shout and the lights going on. He was still half-asleep when the Doctor opened the door.

"Put your shoes on, hurry!"

Jack did what he was told and then followed the Doctor out into the hallway. Mum appeared from her bedroom carrying the holdall that she kept by the door.

"What's going on?" he asked. "Is there a fire?"

"There's an alien spaceship entering the Earth's atmosphere in about two minutes, and if we're still here when it lands then we're probably going to die," said the Doctor. He glanced at Mum. "I assume it's okay to tell the truth, given the circumstances?"

Mum took Jack's hand and started rushing him out of the flat. "What about the neighbours?" she asked.

"They'll probably be fine," said the Doctor. He held up a hand to stop her protests. "There's no time, we have to leave. Now."

They ran outside and down the outside stairs. Jack still didn't know what was going on, but he was frightened enough to move quickly. If his mum was scared then something really bad must be happening.

A tall blue box was standing in the car-park, and it hadn't been there that afternoon. They stopped in front of it and the Doctor unlocked the door, glancing up at the sky as the key turned in the lock. There was a noise from above like thunder, and then...

Then Jack was in a room much larger than the box itself. He opened his mouth to ask about it, and the Doctor answered before he'd found the words.

"Yes, it's a bit bigger on the inside," he said, "sorry you had to find out like this."

Mum took Jack in her arms and the Doctor ran to the machinery in the middle of the room.

"Are we safe?" she asked, breathing heavily.

"I hope so," said the Doctor, pulling a lever on the whatever-it-was. A loud groaning sound filled the air and he visibly relaxed. "We're in flight, heading away from Earth." He walked over and crouched down in front of Jack. "When I said I lived in a caravan, I sort of meant it was a spaceship."

"You lied to me," said Jack, vaguely aware that this wasn't the most important thing that was happening, but quite determined to stick to what he knew for certain.

"For very good reasons."

"I told him to," said Mum. "I was trying to keep you safe."

The Doctor stood up again. "You did keep him safe," he said. "If I hadn't keep showing up on your doorstep you'd have been fine."

"Who found us?" she asked.

"Sontarans. And if they could find you, anyone can, they're not the brightest of the bunch."

"What's going on?" asked Jack, starting to feel less terrified and more annoyed by the opaque nature of events.

"You're dreaming," said the Doctor, "don't worry about a thing."

Mum glared at him. "Doctor, this isn't the time for more lies."

"I'm thinking ahead," he protested. "Do you really want him going home believing in aliens and spaceships?"

She didn't answer, because the air started to vibrate.

"No, no, no!" The Doctor ran back to his machinery. He kicked it angrily. "I put up the shields, what more do you want?"

Jack's mum took his hand, gripping it almost too tightly. "What is it?"

"Dalek transmat beam," said the Doctor, working at the controls. "I'm trying to get out of range."

"You said -"

"The spaceship was Sontarans, the transmat beam is Daleks." He turned to look at them. "Whatever happens, I'll find you."

Mum was shaking now, and Jack didn't think that could mean anything good.

He looked at the Doctor.

"I'll find you," he repeated.

Daleks weren't a fruit after all. They were sort of evil robots with guns instead of hands and without any legs. Jack didn't like them at all, and tried to avoid looking at them whenever possible.

The cell they were in was quite small, and they didn't always get enough to eat. Jack refused to let his mum go without, even when she insisted that she wasn't hungry. Sometimes, when the door opened to let the Daleks in, there were terrible noises from outside. They'd been here for a bit more than a week, as far as Jack could tell, and he hated it.

Still, he had to be brave. He didn't want Mum to worry about him more than she already did, and he believed her when she said that the Doctor would come and rescue them. They just had to wait, she said, because he could spend eternity looking for them and still turn up before teatime.

And then one day, just as Jack was falling asleep for the night, the door opened and amid the blaring of alarms a woman walked into their cell.

Mum was on her feet in an instant, pulling Jack up and holding herself in front of him.

"Oh, come on," said the woman in black, "do you really think I couldn't just kill you to get at the boy?"

"You take one step nearer," said Mum, "and I swear I'll kill you."

"How?" asked the woman, apparently unconcerned. She put her hands on her hips. "Do you want to escape or not?" She pulled her sleeve back to reveal a golden bracelet, which she held up and spoke into. "I've found them, we'll meet you at the rendezvous point."

Mum looked uncertain. "Who were you talking to?"

"Who do you think? Honestly, I don't know what he sees in you. Still, who are we to question the decisions of the hearts?" She smiled down at Jack. "Is this the little one?"

Something loud started happening outside the cell.

"Oops," said the strange woman, "that sounded like the engines. I imagine the whole place is about to go up." She held a hand out towards Jack. "Come on, Aunt Missy's come to save you."

Mum gripped Jack's hand. "Listen," she told him, "you're not to trust this woman. She's a very bad person, but at the moment she's all we have."

Jack nodded and the three of them left the cell. Mum paused in the corridor. "What about the other prisoners?"

"I'll come back for them. Promise." She led them along past burning Daleks and screaming metal. "Someone upset the Oncoming Storm," she said conversationally. "He's very protective of you two, you know. Anyway, I offered to help out."

"Why would you want to help?"

Missy gestured towards Jack. "I didn't want the Daleks hurting another Time Lord. I saw enough of that in the war."

"He's not -"

"The Doctor told me everything. And I do mean everything."

Missy stopped suddenly. "Just a moment." Something exploded and a Dalek shot past them, shrieking, and hit the wall with a loud bang. "This way," she said, carrying on as though nothing had happened.

They rounded a corner and there was the Doctor, looking like the angriest thing Jack had ever seen. His expression softened when he saw them, and he ran towards them. He hugged Mum, kissed her, and then turned to Missy. "The core's about to overload, there isn't much time."

"I said I'd go and free the other prisoners," said Missy, to the Doctor's obvious surprise. "Don't worry about me," she went on, "I've survived worse."

She turned, smiled at Jack, and headed off back down the blazing corridors.

The Doctor took Jack's hand and touched a device on his wrist. "Off we go," he said, and then they were back in the big room that was somehow inside a box.

It was quite disconcerting.

Jack stared up at the ceiling, trying not to fall asleep in case he woke up back in the cell. Or maybe the whole thing was a dream, and he'd wake up back in his own bed. He didn't know which was more likely, so he wasn't going to risk it.

He must have dropped off for a bit, because the next thing he knew his mum was tucking him in and kissing his forehead.

"I didn't mean to wake you," she said quietly. "Go back to sleep."

"Are we going home tomorrow?" he asked.

Mum sat down on the edge of the bed. "We'll go home as soon as it's safe."

"When will that be?"

"Not long now," she said, sounding less cheerful than she looked.

"It'll never be safe," said the Doctor, who had appeared in the doorway.

Mum glared at him. "Don't say that."

"I don't believe in lying to children."

"That's never stopped you before," said Mum. She patted Jack's hand gently. "Do you want some hot chocolate?"

He shook his head. "Why isn't it safe?"

Mum bit her lip and looked at the Doctor. "We should just tell him," she said.

"Tomorrow," said the Doctor, and then he walked away.

Jack didn't really expect the adults to tell him anything, so he wasn't too upset when they both denied that the conversation had ever happened. Anyway, there was something much more exciting to focus on now – they'd travelled in time.

Victorian London wasn't something Jack knew much about, but he was nevertheless quite certain that it wasn't supposed to have lizard-ladies and talking potatoes in it.

The potato - who was called Strax, apparently - stared at him for some time and then said "He's quite a handsome boy."

"Hey, you're improving!" said Mum.

The potato shrugged. "It was a fifty-fifty chance."

"Well," said the lizard – Madame Vastra - "I'm no expert in Time... I mean, in human children, but he does look healthy."

"He's a nice boy," said Jenny, who was married to the lizard.

Jack wasn't sure what to do with all this attention. He moved to hide behind his Mum's dress, but she pushed him forwards so that everyone could get a good look at him.

"You'll inflate his ego," said the Doctor.

Madame Vastra coughed lightly. "I don't wish to be rude, but how long will you be staying with us?"

"About a hundred years," said the Doctor. "Don't worry, we'll clean up after ourselves."

"That's quite a long time," said Jenny.

The Doctor shrugged. "It's not really. Just until he's old enough to take care of himself."

Everyone looked at Jack. He tried to make himself smaller, but it didn't work.

"Surely you don't mean to hide forever?" said Vastra.

"A hundred years is not forever," the Doctor protested. "It's hardly any time at all."

"Well," she replied, "we can shelter you for perhaps a week or so... but you'll have to make other arrangements at some point."

Jack had been through a lot recently. "I want to go home," he said.

The adults looked at each other, but none of them said anything.

"Why can't we just go home?" he went on. He'd been thinking, and had come to a few conclusions of his own. "It was okay until the Doctor started visiting. Maybe if he stayed away it would be safe again."

Somehow the silence became even quieter.

"I really think..." began Madame Vastra.

"We agreed that it was best not to say anything," said Mum. "Isn't that right, Doctor?"

"Why is everyone looking at me?" he asked.

"You appear to be the sticking-point," said Vastra.

The Doctor sighed and then stood up. "Okay, fine. Jack, let's go to the roof."

There were a lot of stars in Victorian London. Maybe some of them had gone out in the future, because Jack was sure there hadn't been this many at home.

The Doctor cleared his throat.

Jack looked at him, and waited. The silence stretched out, and Jack realised he was going to have to deal with the issue at hand. "You're my dad," he said, carefully in case the Doctor took fright.

"Um."

"It's okay," he said, kindly, "I worked it out ages ago. Does this mean I'm an alien?"

"You're not an alien," said the Doctor. "Your mum is, but you're not."

"She's not an alien," said Jack patiently. "Humans can't be aliens."

"It depends how you look at it," said the Doctor. "You're right, by the way, you'd be safer if I stayed away from you."

Jack gave this some thought. "What if you stayed with us all the time?"

"I don't know if I could, I'm not really the domestic type." The Doctor looked at him with a thoughtful expression. "I could maybe manage weekends."

Jack smiled at his dad.

"I mean, it'd be weekends for you," said the Doctor. "I don't know how often it'd be for me."

"That's okay," said Jack. "Time's relative, after all."

"Where did you learn that?" asked the Doctor – his dad – with some surprise.

Jack shrugged. "It's just obvious. Time's relative, and I think space might be as well. Do we have to stay on the roof much longer?" he added. "Only it's a bit cold."

"I suppose we could go back inside."

"Will you read me a story before I go to bed?" asked Jack, taking his hand.

"If you want."

"And will you make me hot chocolate?"

"Don't push it, kid."

They went back into the house, and then, eventually, they went home.