Upon the regeneration of the doctors third incarnation into the fourth,he is thrown into a regeneration crisis where he goes on the lamb from UNIT doctor Harry Sullivan having experienced sudden outbursts of physical strength and erratic memory. Meanwhile a type of advanced ray gun has reported stolen by a scientific research concern calling itself Think Tank. With the doctor in general limbo,his assistant Sarah-Jane Smith goes to visit Think Tank herself to investigate. There are journalistic integrity leads her to follow the project head Miss Winters and her own assistant Jerricho to find a device they called the K-1 robot,which was designed to replace humans in hazardous matters such as in high radioactive environments. During a test of the the device where Sarah's life is threatened,she observes it seems to experience emotions as well. Further investigations by the freshly regenerated doctor and the Brigadier manage to reveal,along with Sarah's own questioning of Professor Kettlesworth,creator of the K-1 robot, that the organization is a massive front for a larger organization called the Scientific Reform Society (SRS),who are convinced that only the intellectually elite should run the Earth.
While UNIT attempt to ambush the event,the robot itself murders an official who holds an important secret: that the SRS are planning to sabotage an international disarmament conference. Hoping a nuclear war will occur that will allow the protected SRS and their robot to lead human survivors in a new society of their rule,it's learned Kettlesworth personally assisted in this himself. However the robot soon kills him,it's own creator,and emerges humanity will have to be destroyed save for Sarah Jane who displayed great kindness towards the robot. This after the doctor and eventually the fail safe device are both able to stop the countdown to the launch of nuclear weapons threatening the planet. In the end the doctor develops a serum that reduces the robot,construction of a biogenic metal,into it's core fragments and the threat is averted. In the end,the doctor invites Sarah-Jane with him as he finally continues his journey in the TARDIS. And the two are joined by a curious Dr. Harry Sullivan.
This is an excellent story to introduce Tom Baker,who'd soon become one of the most famous actors to portray the time lord. Written by Terrence Dicks and edited by Barry Letts,this is essentially still a Jon Pertwee third doctor/UNIT era story. It gives you an excellent chance to see the new fourth regeneration of the doctor very casually dismissing some of the Brigadier's reactionary habits in favor of his own unique (and often more effective) methods. When faced with both the intellectually fascist SRS and the unhinged K-1 robot,Sarah-Jane emerges as her own heroine in the story-having a relationship with the robot reminiscent of a King Kong for the computer age . The story itself,however,deals with the breakdown of all morality in the place of a machine receiving mixed programming by intelligent but completely uncaring individuals. It's Sarah's sense of moral conviction,even when Think Tank leader Miss Winters accuses her of being a reverse chauvinist,that leads to be every bit the triumphant character in this story next to the new doctor.