In an Australian penal colony at the edge of the known world, gentleman convict Hugh Monserrat has risen from convicted forger to trusted clerk of the settlement's commandant. Not long after the commandant heads off in search of a rumoured river, his beautiful wife Honora falls ill with a sickness the doctor is unable to identify. And when Honora dies, it becomes clear she has been slowly poisoned. Monsarrat and perceptive housekeeper Mrs Mulrooney feel suspicious as regards the motives of Captain Diamond, the commandant's second-incommand. In a trice Diamond has Mrs Mulrooney arrested for murder. To save his new friend from the hangman's noose, Monsarrat must go all out to unmask the real killer.