The team investigates when a scuba diver is killed with a gun that was used in a recent murder, while McGarrett gets ready to propose to Catherine.
When a college professor is killed, the Five-0 head back to school with Danny, who goes undercover as the professor's replacement.
When a Russian submarine appears off the coast, the team discover a Russian spy ring hiding in plain sight on Oahu.
Five-O launch a manhunt when a chemical spill allows six inmates, including Kono's husband Adam, to break out of jail.
When McGarrett and Danny pose as drug pilots in a plan to stop a meth epidemic on the island, Danny is forced to break cover and land the aircraft after McGarrett is seriously wounded.
