Adam, Junior and Tani travel to the bottom of the Pacific Ocean when a murder takes place in a remote underwater lab, and the killer may still be on board.

As a dangerous hurricane hits Oahu, Five-0 is tasked with protecting El Diablo, a captured serial killer who is being hunted by an assassin. Also, Tani's civilian safety check during the storm turns deadly and Rachel and Charlie evacuate to Danny's house, where the ex-spouses reminisce.