Five-0 investigates the mysterious death of Billy Keats, a polo player who was decapitated while atop his horse, practicing for an upcoming match. As they investigate the case, they determine that the target was actually supposed to be James Madsen, a teammate who had switched practice times with Billy. Chin Ho and Kono learn that James was kidnapped and held for ransom as a child, and that one of his kidnappers was recently released from prison. Just as Kono arrives to take James into protective custody, a car bomb goes off and nearly kills James' mother, Amanda. McGarrett and Danny track down the original kidnapper, who admits to planting a bomb in James after being paid to do it by an anonymous person online. Later, when Amanda is suffocated in the hospital, McGarrett uses security footage to determine that James is actually the killer they've been after all along. He resented his mother for refusing to pay the kidnapping ransom when he was a child and for hurting his father by having an affair with Billy Keats, so he killed them both.
Meanwhile, McGarrett's mother, Doris, makes an unexpected return. He's forced to come to terms with her coming and going as she pleases, despite Wo Fat still wanting to kill her.