In the conclusion of the four-part story "Planet of Fire," Turlough (Mark Strickson) is reconciled with his past on the planet Trion, while the evil Master (Anthony Ainley) is again caught in one of his own traps. Now the Doctor (Peter Davison) must figure out a way to neutralize the pesky shape-shifting robot Kamelion -- with Kamelion's full permission. Nicola Bryant becomes a series regular as the Doctor's new American companion, Perpugilliam "Peri" Brown. Written by Peter Grimwade, "Planet of Fire, Episode 4" first aired on March 2, 1984. ~ Hal Erickson, All Movie Guide