Bernie Weinstock and his young assistant Robin Bright are a couple of struggling private detectives running their office in a run down caravan in the back garden. Facing debt collectors, they get a stroke of luck when they are contacted by solicitor Roland Tolhurst to do a job that could be worth as much as £30,000. On the orders of his wealthy client Lord Algrave, Tolhurst tells the pair they must locate the whereabouts of six people on a list and give them a sealed envelope, with two curious questions to be asked to prove their identity. They are "Who was the timekeeper?" and "How long before the Indian Princess arrived?" Each person found will earn them £5,000. But as they begin their search, it seems there are others with interest in the case, and one person in particular has murderous intentions to stop them completing the task.

Bernie Weinstock and his assistant Robin have failed in their first task when the first person on the list, window cleaner Frederick Oldfield falls to his death. But they are stunned to discover later that he was murdered, with the only clue being that a figure with a limp was seen leaving the scene of the crime. They set about finding the others on the list, but Joe Galliano, a London gangster named on the list, is keeping a low profile for some reason and making inquiries of his own. However, Robin has located another on the list, the Reverend Arthur Wildblood, leading the duo to travel down to Somerset. But finding the village of Pockling is the least of their troubles as they are not the only ones to make their way down, with a nasty surprise awaiting them.

Private detectives Bernie and Robin's hopes seem to of gone up in smoke with the death of the vicar, but they have further problems when someone cuts the brakes on their car. Realizing that someone has murderous intentions not only on themselves but the people on the list, they desperately try to trace the remaining four candidates. But their quest to find Jean Montpellier at a posh boys school seems to lead to a dead end, until a chance revelation leads the duo to a vintage car museum and across half the world to Haiti, where they encounter the terrifying world of voodoo.

Bernie and Robin find themselves in a real tricky position when they are captured by a voodoo man and his tribe. They manage to get out with their lives, which is more than can be said for Jean Montpellier as another person on the list bites the dust. They seem similarly stumped upon finding the next person Osbert Horatio Faggott until Robin's landlady provides them with the answer, leading them to travel over to France this time to find the film star Greg Winchester. But Soho gangster Joe Galliano is less than happy after one of his goons was killed and sets more of his men out to kill the duo to avenge his death on board the ship across the channel. And it seems they are not the only ones who is handy with a sharp shooter as death continues to stalk the duo.

After Greg Winchester gives the detective duo a ring with a mermaid on to give to his ex wife before he dies in a "on set accident," Robin begins to ponder about the link between the people on the list. Having noticed the tattoo on the vicar's hand, plus the fact that the first victim worked in the merchant navy, the duo discover a connection with the S.S. Mermaid, a ship which sank in 1953. Realizing that Winchester's ex wife Sarah Bedford is in grave danger being next on the list, they head out to Tunisia to find her. But with gangster Joe Galliano also on their trail and out for blood, it proves a very dangerous place for all concerned.

The killer leaves Tunisia believing they have disposed of the detective duo and Sarah Bedford after their car explodes. But luck seems to be in for Bernie and Robin, who smuggle Sarah back into the country as they try to figure out who could be behind the killings and why. Realizing the connection has to be the S.S. Mermaid and it's sinking, they track down Bernie's eccentric cousin Looby, who has an archive of old newspapers, to find out more. And their digging leads to an unexpected avenue as they discover the identity of the "seventh man" in the affair of the S.S. Mermaid. But just who is it behind all the murders?