The ruins of a mysterious hideout in Croatia and the decaying remains of Ireland's Alcatraz contain a haunting past; experts investigate the secrets behind these deserted island structures to uncover how they were built and why they were abandoned. This episode is a mashup of S07E10 "Mystery of the Alien Base," S04E02 "Secrets of Supervillain Island," and S06E09 "Anarchy on Ireland's Alcatraz."