At the height of the London blitz Dr Lennox Collins,pioneer of the new forensic science,is enlisted by DI Wilkins after prostitute Mary Williams is strangled and a swastika carved on her tongue. Wilkins' chief suspect is shy young Anglo-German Wilfred Ziegler,who found the body but Lennox dismisses this as Ziegler is left-handed and the killer was not.The trail leads to the Metropole club,frequented by Mary,where Lennox's new assistant Molly meets the sinister owner Danny Hastings but then two more women are killed in the same way. A rent boy places closet gay actor Ronald Terry at the scene of the last murder though Terry denies seeing anything. However Lennox and Molly are soon investigating Terry's apparent suicide - which they work out is murder. Lennox takes Hastings' DNA but is attacked by men in gas masks and,when Wilkins is taken off the case after bullying Ziegler into a confession, it looks as if Hastings has corrupt police officers in his pocket. Acting separately Lennox and Molly aim to build a case against Hastings but discover another possible suspect and finally have to challenge a government conspiracy in order to bring the killer to justice.