“Bad luck, darling,” said Angela. “It really looks as though he’s the only man who couldn’t have murdered Sir Derek.”

“Then he’s a certainty,” declared Nigel. “Isn’t it true that when there’s a cast-iron alibi the police always prick up their ears?”

“Personally, I let mine flop with a thankful purr,” said Alleyn. “But you may be right. This is scarcely an alibi. Roberts was there; he merely had no hypodermic to give and no syringe to use.”

“And no motive,” added Angela.

“Look for the motive,” said Nigel.

“I will,” said Alleyn. “There’s precious little else to look for. Has it occurred to you, if the lethal injection was given during the operation, how extraordinarily favourable the mise en scène was for the murderer? As soon as a patient is wheeled away they set to work, and as far as I can see, they literally scour out the theatre. Nothing is left—everything is washed, sterilised, polished. The syringes—the dishes—the instruments—the floor—the tables. Even the ampoules that held the injections are cast into outer darkness. If you wanted to think of a perfect place to get rid of your tracks, you couldn’t choose a likelier spot.” He got up and looked at his watch.

“He wants us to go,” remarked Angela calmly.

“It’s only eleven o’clock,” murmured Alleyn. “I wondered if you’d both care to do a job of work for me?”

“What sort of job?” they asked.

“Attend a Bolshevik meeting at midnight.”

“To-night?”

“To-night.”

“I’d adore to,” said Angela quickly. “Where is it? What’s the time? What do we do?”