Peter Davison became the youngest actor to take on the role when he took over from Tom Baker as the Fifth Doctor. Dressed as an Edwardian cricketer with a fresh slice of celery attached to his lapel, his slightly lost and breathless portrayal was a refreshing change from the in-your-face manner of his predecessor. Season 19 also saw the show leave its traditional Saturday tea-time slot for the first time and move to a new twice-weekly evening broadcast.

The Mara, the Terileptils and the Tractators were some of the main monsters featured during Peter Davison's three years in the role, which also saw the show's 'history' being retread as Season 20 featured an old enemy in each story to celebrate this milestone for the series. Also celebrating the show's 20th Anniversary was a specially commissioned 90-minute special, The Five Doctors, which saw the first five incarnations of the Doctor joining forces to battle against Daleks, Cybermen, Yeti and the Master on his home world of Gallifrey.

To celebrate the twentieth anniversary of the show it was announced that every story of Season Twenty would feature an old enemy or element from the Doctor's past. To this end the first story, Arc of Infinity, featured the return of renegade Time Lord, Omega. Snakedance saw the return of the previous season's popular foe, the Mara and the next three stories formed a trilogy with a rematch against the Black Guardian and the introduction of a new companion named Turlough, who was actually an agent of the Black Guardian trying to assassinate the Doctor. The Master returned again in The King's Demons but a planned Dalek adventure at the end of the season fell through due to strike action. However, this was more than made up for when a special ninety minute episode called The Five Doctors was broadcast close to the actual twentieth anniversary as part of BBC's Children in Need Appeal. Featuring the return of Patrick Troughton and Jon Pertwee with the late William Hartnell's role being taken by Richard Hurndall, the production hit a snag when Tom Baker declined the offer to participate, but he was instead represented with footage from the unbroadcast and unfinished story Shada. Packed with old monsters and familiar companions, the story acted as a fitting celebration of twenty years of Doctor Who.

Peter Davison's last season in the role is generally regarded as being his best and also coincided with another clear-out of the TARDIS with Tegan, Turlough and the largely redundant robot companion, Kamelion, all leaving as well as Davison. Coming in were the new larger-than-life Doctor, played by Colin Baker, and a new young American assistant named Perpugiliam Brown who was to make her debut on Planet of Fire, featuring location work in Lanzarote. It was something of a break with tradition to have the new Doctor take over before the end of the season, but was allowed in this instance so as viewers could familiarise themselves with a new actor in the role before he returned the following year. The final story of the Fifth Doctor's reign, The Caves of Androzani, was written by veteran Doctor Who writer, Robert Holmes, making a return to the series after a lengthy absence, and is regarded as one of the best stories in the show's long history.

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