A scuba diver is killed with a gun that was used in a recent murder; McGarrett gets ready to propose to Catherine.
When a college professor is killed, Five-0 goes back to school with Danny, who is posing as the professor's replacement.
Disaster looms in this explosive season finale when, on the eve of Kono's wedding, someone threatens to detonate a nuclear bomb.
McGarrett and Danny get Tani Rey to join the task force when a hacker manages to let out a dangerous arsonist from prison.
McGarrett risks his life when Five-0 attempts to rescue the young girls trapped in the sex trafficking ring they have been tracking; Kono makes a life-changing decision.