The TARDIS lands on an island on the planet of Marinus where it is captured by Arbitan. He reveals The Conscience, the computer that rules over the planet is missing four of the five keys that make it function. The Doctor and his companions must find them. After journeys that take them to many different places, they return with the missing keys only to find that Yartek has killed Arbitan and taken over. He forces the keys from Ian not knowing that one of the keys is a fake. This makes the computer explode and frees the planet from it control.