Disclaimer: created by the wonderful Toby Whithouse and the property of the BBC.

Note: I'm trying to stick to canon as much as possible but it's tricky working out when Hal was on the blood and when not. "Pie & Prejudice" suggests that Lady Mary was killed in the mid-1700s (c.1760) but "The Greater Good" suggested that it was not meeting Mary but meeting the captivating Sylvie "over 200 years ago" that got Hal off the blood for a bit. I wish the lovely BBC people would produce a timeline or something.

The ballroom is thick with the stench of perfume, of perspiring bodies and the flowers used to decorate. The women are dressed in all their finery and the men have matched them. Hal, standing with a glass of wine on the stairs, muses that Hetty would surely love to be here, to pick a victim from the ladies who would coo and fuss over her. But little girls do not come to balls, and although Hetty is all of 200, she will never be rid of that curse.

He puts Hetty out of his mind. He is hungry, and the table below him is laden with a great variety of fancies. There is a blonde in pale blue, her eyes dancing as she moves in the complex lines of the minuet being played. Or a redhead, in unbecoming yellow, on the arms of a Navy captain; but no, the choice of dress is unpromising.

His eyes scan the room again, and now he spots her. She is smiling at something – not laughing, but smiling, her eyes a little downcast beneath the elaborate hairstyle. She has just the right mix, Hal thinks, of innocence and beauty waiting to be shattered. He drains his glass and moves down the stairs.

The dance has ended, and the musicians are pausing for refreshment. The girl is sitting next to two older women, one of whom Hal knows, in a vague, social way. He makes his way to them, bows, and kisses the hand of his acquaintance.

"Lady Marsh, how delightful to see you."

"Lord Harry. Likewise. I trust you are keeping well? Do let me introduce you; my dear Anne, this is Lord Henry Yorke. Lord Harry, the Countess of Strafford, and her daughter, the Lady Mary."

Hal bows again, and does some more hand-kissing. "Lady Mary, may I ask you for the next dance?"

They dance. Lady Mary is an average dancer – competent, of course, but every girl of society is competent – but Hal praises her extravagantly and sets her blushing. When the music stops and he leads her back to her mother she is clearly disappointed not to move into another dance. Hal, turning away to fetch her a drink, smiles to himself. The bait has been taken.

He watches her throughout the evening, and when, alone, she goes outside on to the terrace, he follows. It is a balmy night with the stars bright in the skies and he finds her leaning against a balustrade gazing up at them. There is nobody else around.

She turns and smiles at his approach. "Lord Harry."

Again, he takes her hand and kisses it, but more lingering now and he does not let go afterwards. "Do you like the stars? You can see them better at the other end of the garden, away from the light."

Mary casts a glance back at the house. "I should not ..."

"Nobody will know, if you don't tell them," Hal says, drawing her into the shadows.

She lets out a nervous little giggle. "I suppose not." She glances up at him. "I was … I was hoping we could dance again."

Hal takes her by the waist and waltzes them down the silent, dark grassy avenue. Mary follows his lead, her breath a little short, and he spins her down on to a bench secluded between some bushes. Above them, the sky is full of stars. Mary leans back to look at them, her neck exposed invitingly.

"It's been such a wonderful night," she says.

"One to remember," he says. He leans in, and kisses her. After the first startled moment she responds, inexpert but willing. She tastes of wine and the sweet sherbert she has been eating.

"We … I should not," she murmurs, halfhearted, between kisses.

"Nobody will find out," Hal says. "Nobody at all. Just our secret." He kisses harder, nipping her lips enough to draw the first drop of blood and the taste is as good as he had hoped. He sucks, swallows, moves away from her mouth to her neck and the fast-beating pulse there.

Mary moans; the sound is a trigger. His fangs extend and then he has broken the skin and she is pouring thick and warm into him.

She struggles, a little, for a few seconds, before going limp. He drinks until there is no more to drink and she is still and dead beside him. From a distance, she could be asleep.

Hal takes out a handkerchief, wipes his chin and his hands meticulously, and leaves her on the bench without a backwards glance.