Doctor Who: Summary: The Sea of Death

The TARDIS lands on an island made of glass, surrounded by a deadly sea of acid on the planet Marinus. There, the Doctor and his companions meet Arbitan, keeper of a machine called the Conscience of Marinus. Arbitan enlists them to find the five hidden keys that power the machine, which can control minds across the planet. He warns that the keys must be recovered before the alien Voord and their leader, Yartek, seize them and use the Conscience for conquest. Each key is scattered in a different location, forcing the group to split up and begin a dangerous quest across time and space