The death of a polo player who literally loses his head is investigated. Meanwhile, McGarrett’s mother pays him an unexpected visit.

Five-0 investigates the murder of a traditional Hawaiian fighting master. Meanwhile, McGarrett hires a private investigator to follow his mother.

Five-0 must free McGarrett's mother, Doris, when Catherine informs them that she was captured and is about to be executed after attempting to free Wo Fat's imprisoned father. Also, Chin loses his appeal to have Sara's adoption delayed, and she must move to Mexico.

When the CIA informs McGarrett that his mother has gone rogue on her undercover mission and may have killed her partner, Steve goes solo to Mexico to bring her home and prove her innocence.