The Doctor and Sarah make an unscheduled trip to Karn, a dark and story planet close to Gallifrey. He blames the Timelords for messing with his Tardis and forcing him to do their 'dirty work' as they did with the Daleks. At first he refuses to get involved but Sarah soon finds a field of crashed ships and a headless creature called a Mutt. The Doctor realizes the head was dismembered after the crash and deliberately. This is enough to get him interested in what's going on.
When it begins to rain, the Doctor and Sarah seek shelter from the only 'civilization' they can find, a large majestic mansion on top of a hill, lightening flashing behind it. Inside they meet Solon, who immediately praises the Doctor for having such a beautiful head. Sarah claims to have liked his previous silver model. When the Doctor thinks he recognizes the clay bust Solon is working on, Solon quickly covers it up.
There is also another group on Karn known as the Sisterhood. They have a connection with Gallifrey who knows of the Elixir of Life the sisterhood secretly protects. But the flame which is the catalyst for the elixir is dying and sisters are convinced the Timelords sent the Doctor to steal what remains. They then teleport the Doctor away from Solon, who they think he is conspiring with. Finding out Solon has been less than honest, Sarah sets out to find the Doctor. She succeeds in infiltrating the sisterhood and saving him from being burned alive, but a flash from high priestess Maren's ring takes her eyesight away.
Despite not trusting Solon, the Doctor takes Sarah back to have her eyes checked. Solon tricks the Doctor into thinking Sarah's eyes can only be restored with some of the Elixir. After he sets off, Solon sends his servant Condo ahead of the Doctor to warn the sisterhood the Doctor plans on stealing the Elixir. He hopes they will give him the Doctors head in exchange for the warning. While he's occupied, Sarah makes a discovery of her own. Solon has the brain of Morbius in his laboratory. Morbius is a Gallifreyan who was executed for his crimes of treason and attempts of galactic conquest.
Realizing he's been tricked by Solon, the Doctor offers to help the Sisters restore the flame that brings the Elixir of Life. Without it, they will soon perish. The Doctor also tries to assure them the Timelords did not send him to steal it. He's successful in reigniting the flame, but the Sisters abide to the agreement they made with Solon and send the Doctor back to him, bound.
While he was away at the sisterhood, Solon was successful in placing the brain of Morbius inside a hodgepodge body created from spare parts of those who crashed on the planet, including the arm of his servant Condo. Morbius plans on continuing his reign of terror, but the Doctor challenges him to a mind contest. This fries his delicate brain and Morbius tries to escape but is hunted by the Sisterhood who chase him with torches and ultimately over a cliff.
Next: Eldrad Must Live