The Fourth Doctor

In Baker's first season, Season 12, the Doctor's adventures centred around space station Nerva, accompanied by Sarah Jane Smith and Harry Sullivan. They would face the Sontarans, the Cybermen, and the Daleks, witnessing the birth of the latter at the hands of their creator, Davros.

Harry left the TARDIS at the beginning of Season 13 after Terror Of The Zygons, but would appear one last time in The Android Invasion. Sarah Jane was forced to leave after Season 14's The Hand Of Fear as she would not be allowed to go with the Doctor to his home planet, Gallifrey, after being summoned by the Time Lords. After defeating the Master in The Deadly Assassin, he is joined by Leela after they meet in The Face Of Evil. The Doctor then inherits a mobile computer in the shape of a dog, K9, in The Invisible Enemy in the following season.

Leela leaves the TARDIS with K9 at the end of The Invasion Of Time but the Doctor has already built K9 Mk II! Season 16 sees the Doctor set on a quest by the White Guardian, to retrieve the various parts of the Key to Time. He is assigned an assistant in Time Lady, Romana. after completing their mission, Romana decides to regenerate at the beginning of Season 17. The Doctor would once again face the Daleks and their creator, Davros, in Destiny Of The Daleks.

In Baker's final season , season 18, the Doctor and Romana are joined by boy genius Adric, before Romana decides to leave the TARDIS with K9 in Warriors' Gate. Nyssa arrives, after the Master kills her father in The Keeper Of Traken, and then they meet air stewardess Tegan Jovanka in Logopolis. Meanwhile, the Doctor is being followed by the Watcher who merges with him after a fatal fall from a radio telescope while attempting to defeat the Master again.