Thousands of years ago, on the planet Kastria, a criminal named Eldrad is sentenced to death for destroying the barriers that protect the planet from the Solar Winds. The pod containing the criminal is destroyed, but his hand survives. In the present day, the TARDIS appears in a quarry and the Doctor and Sarah Jane are caught in an explosion. Sarah is unconscious, but she contacts the fossilized hand of Eldrad and comes under the influence of its ring. In the local hospital, she and a Doctor Carter (Rex Robinson) are both hypnotised by the ring.
Sarah heads for the nearest Nuclear Reactor, the Nunton Complex (played admirably by the Oldbury Nuclear Power Station in Gloucestershire) and breaks into it with the hand. It thrives on radiation and begins to regenerate. It can even move around on its own, providing the image of a creeping hand, spooky in any movie. The head of the reactor complex, Professor Watson (Glyn Houston) stays at his post even as the reactor begins to go critical. Doctor Carter, possessed, tries to stop the Doctor from getting to Sarah Jane and falls to his death. Eventually, the Doctor does reach Sarah and takes her to the infirmary.
The ring takes control of a nuclear technician named Driscoll (Roy Boyd) who takes the hand into the core. RAF bombs only make it worse—Eldrad regenerates into a fully humanoid shape (Judith Paris, who is tall and drop-dead gorgeous, with her blue eyes matching her crystalized blue skin). She learns why the humans want to destroy her and convinces the Doctor to take her to Kastria, explaining that she built the solar barriers and they were destroyed in an interstellar war.
The TARDIS arrives in Kastria in the present—150 million years after Eldrad’s time--to a barren, frozen world, where a few survivors apparently live deep below the planet’s surface. Eldrad is caught and apparently destroyed by a trap but emerges in his true masculine form of old (Stephen Thorne). He goes into a rant upon seeing King Rokon’s recorded image and admits that he destroyed the barriers in rebellion against him. The pre-recorded image reveals that the Kastrians accepted extinction over a miserable subterranean existence and even destroyed the genetic banks so that Eldrad, if he returned, would be King of Nothing. So Eldrad decides to build his empire on Earth, and the Doctor trips him up and hurls him into the abyss.
Sarah is fed up with everything that has happened to her travelling with the Doctor and packs her belongings in the TARDIS. The Doctor receives a summons from Gallifrey and says he cannot take her with him. Now, she’s upset. The Doctor returns her to Earth. In a touching, but restrained, scene, she tells him not to forget her, and she watches the TARDIS dematerialize, then realizes she is nowhere near her home. She walks off, whistling.
Sarah was originally supposed to be killed off in a French Foreign Legion story that never happened. Sladen was happy not to be killed off or married off; she wanted to leave on her own terms. In fact, she and Tom Baker wrote the farewell scene together. The story was praised, and her acting was singled out. Sarah Jane Smith appeared in eighteen stories with the 3rd and 4th Doctors, in the Five Doctors story in 1983 and Dimensions in Time in 1993. And, of course, she starred in the spin-off series The Sarah Jane Adventures with K-9 (2007-2011), in which the 10th and 11th Doctors guest-starred. She changed the Doctor’s companion from a helpless hysteric to a feisty, opinionated equal to the Doctor, and you can see bits of her in every companion who followed. I would add that Tom Baker contributed to this transformation, as he saw much better than the BBC suits how much the main relationship in the series would be enhanced if they could ditch the scream-queen.