William Hartnell, the first Doctor, stars in this series concerning Marinus, a planet where the sand is glass and the sea is acid. The Tardis is captured by Arbitan, the keeper of the Conscience of Marinus, a machine which controls the island. 4 of the Conscience's keys are lost, though, and the Doctor must go in search of them on various worlds. When he returns, however, he finds Arbitan murdered, which complicates matters thoroughly.