There have been a series of murders of young women, and the police commissioner is demanding results from the detectives. A common feature is that they all took place at 3 am, and the victims were leaving clubs whose names seem to follow a pattern -- P, L, A, Y - What next? But then an aged waterman puts the detectives on to another common factor -- the dates were all of high tide on the River Thames. A woman is set up as a decoy at a club on the next expected date, but she is taken away by taxi without incident. Without incident, that is, until it is reported that the taxi was stolen earlier. Will the detectives find her, and the killer, in time?