1976
When he was summoned back to Gallifrey, the Doctor was forced to take fan favourite Sarah Jane Smith back home, at the end of The Hand of Fear. Although both were reluctant to say good-bye to each other, thanks to the nature of travelling through time and space, it wouldn't be her last adventure. For the first and only time on television, The Deadly Assasin saw the Doctor have a companionless adventure, but it is well known for a number of reasons. It also introduced the idea of each Time Lord having only twelve regenerations, saw the return of the Master, and is not just the only serial in which every character is from the same race (it is set entirely on Gallifrey), but apart from a female computer voice, the cast is also entirely male.
1977
The idea of the Doctor travelling in his TARDIS alone was something that didn't stick, and so two new companions were introduced. Having descended from the survivors of a crashed survey team and raised as a warrior, Leela was introduced in The Face Of Evil. Although uneducated she was highly intelligent, brave, fierce, and would often have to be restrained by the Doctor when looking for a fight. Apart from being such an active rather than passive companion, she is also often remembered by male fans for wearing loincloths. The Invincible Enemy also introduced K9, the loyal tin dog invented by Dr. Marius, but who joined the Doctor and several companions for a number of adventures in the TARDIS. Quite often literally 'in' the TARDIS, as the the Doctor was occasionally known to make him stay behind when exploring.
1978
The show's 16th series which began broadcasting this year featured a series long arc, known as the Key To Time. Recruited by a being known only as the White Guardian, the Doctor was accompanied by fellow Gallifreyan companion Romana on a quest throughout space and time in order to find and assemble the key's six pieces. Throughout this prolonged adventure the Doctor celebrated his hundredth serial, The Stones of Blood, as well as The Pirate Planet, the first to be written by Hitchhiker's Guide To The Galaxy author Douglas Adams. Although his later story Shada wouldn't be completed due to industrial strikes, he co-wrote City of Death under the name David Agnew, and also acted as script editor for the show's 17th series.