In the conclusion of the four-part story "Castrovalva," the TARDIS crew is trapped on huge Block Transfer Computation, which the Master (Anthony Ainley) has disguised as an arboreal planet. Knowing that he and his companions must escape this false world lest the Master succeed in his latest bid to take over the Universe, the Doctor (Peter Davison) must first rescue the brainwashed Adric (Matthew Waterhouse) from the villain's evil influence. Written by Christopher H. Bidmead, "Castrovalva, Episode 4" originally aired on January 12, 1982. ~ Hal Erickson, All Movie Guide