#7 - "Robot"
Even though the Doctor has only just regenerated, the Brigadier enlists his help in discovering who (or what) has been stealing high-tech parts from a number of scientific facilities, and to what purpose. Meanwhile, Sarah Jane Smith investigates Think Tank, a high-tech facility that has gone unaffected by the recent break-ins. There, she discovers the titular robot. It's mean to be a pacifist, but is someone playing havoc with its programming? Yes. Yes they are.
The good and the bad of Tom Baker's introduction is that it's very inauspicious. The world isn't in terrible peril so much as there's an extremist group who is being, well, very extreme. Yes, our heroes' lives fall into some jeopardy, but there's never any sense that they, or the world, are in any real danger.
Plus, when the robot grows to enormous proportion, your interest will shrink into nothing. The longer the episode goes on, the less compelling it becomes.
Fortunately, Tom Baker is character-definingly amazing. Plus, Liz Sladen's Sarah Jane never disappoints, and Ian Marter is a welcomed addition to the team as the affable-if-bumbling Harry Sullivan.