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?