The 20th season stars Peter Davison, returning for a second year as the Fifth Doctor. In "Arc of Infinity, " Omega's latest plan to escape his anti-matter imprisonment causes his essence to merge with that of the Doctor. Will the Time Lords be forced to destroy both of them to save the universe? Then, Tegan (Janet Fielding) is once more possessed by the sinister, serpentine Mara, and the Doctor and Nyssa (Sarah Sutton) must find a way to stop it from crossing over into the real world without it costing their companion her life, in "Snakedance." The latest scheme by the sinister Black Guardian (Valentine Dyall) to gain revenge on the Doctor sends the Time Lord to Earth where he encounters the unflappable Brigadier Lethbridge-Stewart (Nicholas Courtney) and the mysterious Vislor Turlough (Mark Strickson), in "Mawdryn Undead." The Doctor and company find themselves trapped on board a starship loaded with plague victims and bound for the space station "Terminus." Next, an interstellar "sailing ship" race is in reality the Black Guardian's ultimate plan to destroy the Doctor and Turlough's final chance to decide which side he's on, in "Enlightenment." "The King's Demons" finds the Doctor tasked with keeping the Master (Anthony Ainley) from taking over medieval England with the aid of a shape-changing robot named Kamelion (voiced by Gerald Flood). Finally, celebrating two decades since "Doctor Who's" 1963 debut, "The Five Doctors" is an exciting adventure involving several of the Time Lord's incarnations, a slew of companions, the Master, the Cybermen, and more. Patrick Troughton, Jon Pertwee, Richard Hurndall (taking over for the late William Hartnell as the First Doctor), Tom Baker (appearing via then-unused footage) also star.22 episodes and an anniversary special on 9 discs. 9 hrs. Standard; Soundtrack: English; Subtitles: English; featurettes; interview; more.