When an escalating gang war spills into the stands of the local high school football game, injuring several bystanders and killing a gang leader, Five-0 uncovers an unlikely partnership between Samoan gangs and a mainland mob.
McGarrett places himself in the middle of a hostage situation on the USS Missouri. The hostage-taker is an ex-Navy SEAL suffering from post-traumatic stress disorder, who is the prime suspect in the murder of his wife.
Danny's ex-partner from the Honolulu P.D., is found in a luau barbecue pit. Danny demands the team must take his word the man was incorruptible, although evidence piles up ha was involved in corruption surrounding the Mexican Ochoa cocaine cartel. Internal affairs Sergeant Cage, who wrecked Chin's reputation, is on the case but fails to shield it as HPD Detective Ahuna discretely joins the Five-O investigation. The deeper they dig, the dirtier everyone looks. Jailed drug dealer Sang Min holds a trump card, so Danny bids on it.
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.
A costumed comics convention superhero fell to his death, pushed from his hotel balcony, but only as the victim of a drunk rooms mix-up. The team must work out how a ransacked room, no robbery, compromising pictures and adultery fit together. Steve rightly mistrusts a self-styled CIA-agent, works out she's a mere analyst and not even assigned, but decides to collaborate as she has indications his father's killer may have worked for Yakuza boss Wo Fat. Danny grumpily started dieting for his worried brat's sake.
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.