The Doctor and Jo visit the Master, now held in captivity on a small island prison, after being captured by UNIT (at the conclusion of The Dæmons). The Master is being held indefinitely and is the only prisoner. He is watched by CCTV and the island is patrolled by armed guards, trained to resist the Master's hypnotic powers, and even protected by minefields. He claims to have reformed but refuses to reveal the location of his TARDIS. As they depart, the old-school patriotic governor, Colonel Trenchard, tells them that some ships have been mysteriously disappearing. The Doctor cannot resist investigating and he and Jo are soon attacked, while examining a Sea Fort, by an underwater Silurian. This man-sized bipedal lizard is called a 'Sea Devil' by a crewmember who's been driven half mad. They escape to the nearby naval base, HMS Seaspite run by the efficient Captain John Hart; despite the Doctor's eccentric behaviour (such as claiming to have known Nelson personally), an alliance forms. Hart is in charge of the adaptation of the sea fort for use as a SONAR testing station.