William Arrowood, Victorian London's less salubrious private detective, is paid a visit by Captain Moon, the owner of a pleasure steamer moored on the Thames. He complains that someone has been damaging his boat, putting his business in jeopardy.
Arrowood and his trusty sidekick Barnett suspect professional jealousy, but when a shocking discovery is pulled from the river, it seems like even fouler play is afoot.
It's up to Arrowood and Barnett to solve the case, before any more corpses end up in the watery depths . . .