Suave private investigator, Norman Conquest, (Tom Conway) intercepts a secret message and meets a beautiful but foreign blonde lady, Nadina Rodin (Eva Bartok), in room 605 of the Park Plaza hotel. But when Conquest wakes up in the room the next morning he is lying next to a dead body. With the mysterious blonde nowhere to be seen, Conquest soon becomes the police's number one suspect with Inspector Williams (Sid James) following his every move.
In order to clear his name, Conquest enlists the help of Pixie Everard (Joy Shelton), but the going gets rough when he discovers that the murder is connected to a stash of stolen diamonds. As gun-happy gangs of communists and Nazi sympathisers turn up the heat, Conquest has to solve the murder whilst staying one step ahead of both the gangs and the police.