Hah, het warning! Or, warning for not very explicit sex.
In the end it's not her fault. Whatever happens can be blamed solely on Sherlock, and his ability to make stupid and impossible things sound reasonable.
"I need your help," is what he says when he's been banging on her door in the middle of the night. Mary is wearing her morning gown tied tightly over her night shirt, and he just looks at her. "No time for your propriety, lives are at stake!"
"Where is John?" Mary asks, because he was helping Sherlock with his case, and he's not here now.
"Safe," Sherlock says. "I need you to put on this and come with me, no time to lose." He holds out a dress to Mary, and she sighs and closes the door to put it on. It's a bit more revealing than she usually wears, but nothing too bad, and Sherlock looks approving when she comes out to show him. He takes her hand and drags her out to a hansom cab waiting just outside the door.
*
When they get to their destination Mary sees John and Irene waiting for them. Irene is wearing a dress similar to her own, and John looks grim.
"Holmes, I'm sure this isn't necessary," he says.
"Nonsense, besides, she already agreed to help, and you wouldn't presume to tell Mrs Watson what she can and can't do, would you?" Sherlock says, smiling. John sighs. He looks at Mary.
"Did you agree to this insane plan?" he asks her.
"It seemed reasonable," she says, and smiles. Truth be told, if she wasn't already yearning to touch Irene she probably wouldn't have agreed to act with her, even if it was to catch a killer, but as it was, playing prostitutes (and apparently this man likes to have to females perform for him, so to speak, and then kill them) was an opportunity she couldn't resist.
*
After, she and John make love in an alley near the house where the wealthy man lived. She feels almost like the lady of the night she's pretending to be, except John kisses her, and tells her how much he loves her, and she wonders if Irene and Sherlock are doing the same somewhere. Wishes they were close by, next to her and John, close enough to touch. She wonders if John wishes the same, and if she will one day find the courage to ask him.