Ep 9. Logopolis, Episode 4
- March 21, 1981
As the Doctor and his companions prepare to bring a broken TARDIS to Logopolis, an advanced civilization that can repair the advanced technology of the Time Lords, they find themselves being pursued by a mysterious figure known only as the Watcher. Upon reaching Logopolis, the Doctor discovers that the planet is in grave danger and that he must work with the resident mathematicians to prevent the universe's destruction. However, when a traitor is discovered among the Logopolitans, the mission becomes even more perilous. Meanwhile, a sinister force is also closing in on the TARDIS, threatening to put an end to the Doctor's travels forever. The Fourth Doctor and his companions face their greatest challenge yet in this thrilling episode of Doctor Who.