McGarrett's father's toolbox is stolen, and his sister is kidnapped for investigating their mother's death. McGarrett soon discovers that his father was investigating a suspected Yakuza leader before his death.
The island's leading tsunami expert is kidnapped on the same day that ocean reports indicate that a huge tsunami is headed to Hawaii. However, Five-0 soon suspects that the report is a hoax and is related to the kidnapping.
While Five-0 races to stop a mad bomber before he kills again, Danny gets an unexpected visit from his little brother, Matt, a fun-loving, Wall Street broker. The family reunion is cut short, however, when Danny discovers the real reason that Matt is in Hawaii.
Five-0 must unravel the mystery of why a mild-mannered sci-fi fan wearing a cape was given truth serum before being tossed out of a 21st-story window. Meanwhile, McGarrett gets an unexpected visit from a CIA agent who shares his personal interest in tracking down the Yakuza who had his parents killed.
The team investigates the death of a commercial fisherman whose body was found in an unexpected place, while the imminent death of Chin Ho's aunt could reveal surprising new details about the stolen drug money.
The team discovers that a dead salvage diver was looking for more than sunken treasure, Chin Ho discovers that Kono accessed his police account, and McGarrett becomes curious about what's delaying the DOD findings about his father.
While Five-0 investigates the death of a muckraking reporter, Jenna returns to ask McGarrett for help finding her fiancé in North Korea--which is actually part of Wo Fat's revenge plot--and Joe White leads the mission to bring them home.
A team of drug dealers kidnaps a busload of school children in order to get Five-0 to return the drugs they confiscated, while Joe White goes on a personal mission to uncover the identity of "Shelbourne" before Wo Fat does.
As the gang gets ready for Chin Ho's wedding, Five-0 discovers that a man shot in a military bunker was in Hawaii to search for his missing son, and Steve begins to suspect that Joe is hiding secrets about the investigation from him.