While Danny chaperones Grace's winter formal, terrorists seize the venue and hold everyone hostage in order to kidnap a diplomat's son. Also, Danny and Grover are shocked to learn that Will is Grace's secret boyfriend.

Five-0 investigates the 10-year-old cold case of a missing teen when a boy suffering from PTSD undergoes hypnosis and realizes he may have witnessed her murder. Also, Danny's sister Bridget visits Oahu for a business conference, and he suspects that she may be getting too close to a co-worker.