Someone wants to kill Mister No’s most recent client, the wealthy Carlos Caldero. A real enthusiast for underwater exploring, Caldero is determined to leave on his luxurious motor trawler “O Rei”, on which his nephew and niece Almeido and Marinha have also embarked (Marinha is immediately attracted to Mister No). But, on board the ship, the attempts on his life multiply: who is the culprit?