This is the sort of thing that happens when you spend too much time thinking about what would happen if the Doctor met characters from your other favorite shows.
The body was fresh. At least, that was Lanie’s best guess. “Drained dry,” she told Beckett. “As far as I can tell, your victim’s got no blood.” An officer pointed out a man among the looky-loos and said he’d asked to speak to the lead detective. Beckett followed the uni to the tape line and gave the guy a once-over. Tall, white, thirties. Unruly hair. Brown pin-striped suit, long trench coat. Sneakers. Interesting.
“You’re in charge here?” he asked as she approached. He had a London accent.
“Detective Kate Beckett. Who are you?”
"I’m the Doctor,” he replied, as though it meant something. “Tell me, Detective Kate Beckett, do you have a bloodless murder victim in that alleyway back there?”
Beckett narrowed her eyes. “How did you know that?”
“Because somewhere in this city, there’s a Plasmavore hiding in plain sight, just waiting to drain someone else dry. You’ll never find him with fingerprints or forensics, and if you don’t find him soon, the Judoon will sweep in, and believe me, you do not want them scanning first and asking questions later. You need an expert, Detective. You need me.”
Beckett laughed. “In this job, Doctor, I can believe a lot of things, but that line of bull you just fed me? Not one of them.”
The stranger looked grave for a long moment. Suddenly, a wide grin lit his face. “Oh, we’ll see about that, Kate.”