After a college professor is murdered, Danny goes undercover as his replacement and Eric poses a college student. McGarrett gives advice to a young man after his dad returns from jail and wants to get back in his life.
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.
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.