Astrid only leaves the lab for a minute, but when she gets back Peter is nowhere to be seen. She stops at the top of the stairs and scans the room, wondering how he's left without her knowing. She frowns, trying to think, before carrying on down the stairs where, at the bottom, she notices the beakers all over the floor (good thing they're shatter-proof, she thinks). Stooping to pick them up she realises it isn't just beakers on the floor, there's bits of metal, a computer keyboard, a lamp, a packet of chips – all of the things that had been on the table when she'd left the room. She holds back a "What the hell?" as best she can, but it still escapes from her mouth anyway. It looked like someone had pushed them all on the floor and Astrid tuts, annoyed because it's always her that clears up after every case.

She wanders over to the trash can with the chips and it's then that she realises Peter hasn't left. He's sitting on the floor leaning against the deprivation tank with his head in his hands. "Peter?" She takes a few steps towards him. "What's wrong?" He doesn't answer. She walks slowly down the steps in trepidation, knowing that her question was a stupid one (her mother always said she asked the most obvious questions, but Astrid thought the obvious questions were a good segue way into the harder ones).

Peter looks up, rubbing his hands through his hair, but doesn't say anything. His face is etched with hurt and guilt and pain, and the fear it strikes in Astrid makes her feel like all her organs have disappeared and there's a great big hole inside of her. She kneels down and puts her hand gently on his knee, squeezing a little. "Olivia?" she asks. Peter closes his eyes and nods.

"I should have known," he says, his voice thick.

"Oh, Peter." She sits down beside him. "She fooled us all, you know."

"I should have known," he repeats.

"She was good at her job, Peter." Astrid waves her hands around to emphasise the point. "This alternate Olivia was genetically identical to our Olivia, they think and feel the same way and you know how good our Olivia is at her job, you know she will stop at nothing to get results. Their Olivia was exactly the same and every bit as good."

"I still should have known," he replies through gritted teeth, like he hasn't even heard her. Astrid sighs and takes his hand (even when she's right no one pays attention). "What am I gonna tell her?"

"You'll tell her we did everything we could once we knew the truth."

"I mean," his voice is strained and he pulls his hand away. "What am I gonna tell her about us? 'Oh sorry, Liv, while you were trapped in an alternate universe I was screwing your doppelgänger thinking it was you, but it's OK now you're back we'll just pick up right where we left off'?"

The burst of anger scares Astrid and she recoils slightly. She's seen Peter angry at suspects, she's seen him angry at Walter, but she's never seen him angry at himself and it's on a completely different level (it reminds her of her father). She'd known he and Olivia had become much closer when they'd returned from the alternate universe, but to hear it put so crudely shocks her; it's not something she expects from the Peter she's come to know and care for.

After a silent minute Peter says, in a voice Astrid is more used to hearing, "You know, I did know. I knew something wasn't right." Astrid looks at him but Peter can't meet her eyes. "I felt it, I even mentioned it to her!" He scratches his head violently, nervously; he throws his hands about in frustration. "What kind of an idiot am I? She was a completely different person! She hesitated when she shouldn't have, she had a completely different approach to her work, I mean what kind of a person am I that I don't even notice that the woman I fell in love with wasn't even here any more?" He looks at Astrid now, his eyes pleading for an answer, for Astrid to make it all right.

They sit in silence for a while, partly because Astrid doesn't know what to say and partly because she is wrestling with whether to make her own confession. Peter reminds her of the many broken people she has seen in her life, none of whom she could fix, or help, or bring back from the brink. (It's like they get smaller and smaller until you haven't noticed they've gone.) In the end, the not knowing what to say, not knowing how else to help Peter, forces her to come clean too.

"I knew as well." Her voice is soft, barely above a whisper, but it is what Peter needs to hear and so he hears her loud and clear. She looks him in the eyes. "I..." she swallows hard and continues slowly. "I knew too. About Olivia. I had a feeling something had happened. But I didn't know what." Peter nods. "I mean, I didn't know how to express it, I didn't know how to bring it up. What would I have said?" Her eyes are wide, hopeful that this declaration won't invoke another burst of Peter's anger. She's already angry at herself.

"What," Peter coughs, clearing his throat of emotion. "How did you know?"

"It was the way she was with Walter," Astrid replies with a hint of scorn for the woman who tricked them all. "She didn't treat him like the man she had so much history with, y'know?. Like a man she can't quite forgive, but loves too much not to." (Like Astrid was with her father.) "It was a subtle difference, but..."

"You noticed," Peter finishes and smiles. "You always notice, Astrid."

"It wasn't enough just to notice though, was it?"

"No," Peter sighs.

Mindful that she has just turned this situation into one about herself, Astrid takes Peter's hand again and holds it softly. "She was still extremely good at her job, Peter; and even though we were slightly sceptical they were all minor differences we could have put down to crossing universes. Who knows what kind of an effect that has on a person?"

Peter looks at her pointedly. "The kind of person who's done it before."

Astrid's expression is doubtful. "Peter, you were seven years old and completely unaware, there's no way you could have seen this in anyone else."

He chews his lip for a moment, thinking.

"I still have to tell her what happened."

Astrid studies Peter now and he looks like a man with a burden lifted. Instead of getting smaller, he looks exactly the same. Maybe Astrid brought someone back for once. (She wishes her father could have seen it happen.)