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.

Tani and Junior must plot an escape for themselves and several civilians after they become trapped inside a deadly tunnel collapse, while Five-0 investigates the cause and culprits of the cave-in. Eddie the dog uses his field experience and sniffs out a bomb that has been planted in McGarrett’s home.

McGarrett and the team investigate when a deadly hit-and-run involves a driverless car carrying heroin, and what could be a new means for delivering drugs. Also, Tani and Quinn are stuck with a pair of YouTubers who are brought on a ride-along with Five-0.

When Five-0 investigates a tourist helicopter that was shot down by a missile, they trace the stolen artillery to a ghost from McGarrett’s past. After Adam’s ordeal in rescuing Tamiko and reconnecting with the Yakuza, he makes a bold decision about his future with Five-0.

Harry Langford helps Five-0 investigate a series of murders that follow the plot of a fabled unpublished crime novel from the 1920s. Also, Tani and Junior's friendship takes a meaningful new turn.