The Behavioural Analysis Unit is called in on a Denver case where a home invasion led to the slaughter of an entire family.
The team investigates a shipwreck carrying exotic animals that now threaten the native wildlife on Oahu; Alex sustains a career-ending injury; Kai asks a fish and wildlife agent out on a date.
A special personal mission sends McGarrett's Aunt Deb (Carol Burnett) to Oahu; unstable Japanese WWII bombs hidden on the island are stolen.
On the eve of Chin Ho's wedding to Malia, the team's investigation into a man left for dead in an abandoned WWII bunker crosses paths with Captain Fryer's case of a Jane Doe.
Blindsided by an attack unlike anything they've experienced before, a confused Marcus and Sam race to save the sub and defend the island.