The one with talking rocks.
Arriving back in modern-day England, the TARDIS lands in an actual quarry for a change. The Doctor thinks it's all very nice that there's a big siren to welcome them and a guy way in the distance seems to be happily waving at them. The quarry, of course, immediately explodes in a routine procedure, and Sarah Jane is buried under the rubble.
Before being rescued, Sarah touches what seems to be a fossilized hand and is taken over by the spirit of Eldrad, a Kastrian criminal punished by disintegration — but whose hand has survived, and whose spirit lives on inside the ring on its finger. Sarah takes the ring and goes to a nearby nuclear research station, with the Doctor and friends arriving a few minutes later. She uses the energy to recreate Eldrad's body, absorbing all available nuclear energy. The Doctor and the resident lead scientist, Dr. Watson, convince the RAF to bomb the place (after evacuating everyone). Eldrad isn't bothering, and appears in the form of a very beautiful, very confused woman. When she learns that hundreds of millions of years have passed since she crash-landed on Earth, she asks the Doctor to take her back in time, but he refuses. He does, however, agree to take her home to Kastria in the present. But when they arrive, there is only a dead planet and a mocking message from the last king of the Kastrians. Eldrad (now in a male body) is furious, turns out to be evil after all, and forces the Doctor to take him back to Earth so that he can rule there instead. The Doctor and Sarah trip Eldrad using the Doctor's scarf and the crystalline being falls into a crevasse and shatters.
Returning to the TARDIS, the Doctor is summoned to Gallifrey and, as he thinks that he can't take Sarah with him, he gives her a lift home to South Croydon. She was planning on leaving anyway, so although the farewell is bittersweet, she doesn't mind too much... only for her to realise, as he departs, that she doesn't recognise the street where he's left her. And that it's probably not even South Croydon. (As we find out much later, it was, in fact... Aberdeen.)
- Be Careful What You Wish For: Sarah threatens to walk out on the Doctor, only for the Doctor to ask her to leave because he's going back to Gallifrey. Even though Sarah would love to see the Doctor's home planet, he sticks to his decision.
- BBC Quarry: Played with, as the TARDIS lands in... an actual quarry.
- Bizarre Alien Biology: Inverted. Sarah Jane is able to shake off the effects of a poison gas, because it is designed to be deadly to silicon-based Kastrians.
- Brainwashed and Crazy: Eldrad must live!
- Brick Joke: We don't discover where the Tardis left Sarah until the New Series episode "School Reunion".
- Call-Back: This isn't the first time the Doctor hypnotised Sarah.
- Corpsing: Eldrad starts having a Villainous Breakdown and begins Milking the Giant Cow, Chewing the Scenery, and generally foaming at the mouth to extents that are aesthetically indefensible, and when he suddenly raises his voice, Tom Baker giggles before turning it into an in-character Grin of Audacity.
- Cute Monster Girl: Eldrad's shapely female form (which is visibly created by putting an actress in a latex catsuit, for extra points).
- Disney Villain Death: Eldrad falls down a deep black pit after being tripped up with the Doctor's scarf. Being a being of stone, the Doctor suggests he may have survived..
- Before him, the nuclear reactor worker that tried to kill the Doctor, though he wasn't really a villain.
- Distant Prologue
- The Dulcinea Effect: The Doctor seems a lot more willing to help Eldrad when s/he looks like a sexy blue chick◊ rather than a hulking rock dude◊. Sarah Jane is rightly skeptical of Eldrad the whole time. Meanwhile, Eldrad was actually exploiting this trope.
- Evil Is Hammy: The more Eldrad reveals his megalomania, the hammier he gets.
- Evil Sounds Deep: Eldrad's Kastrian form.
- Face Death with Dignity: Eldrad had destroyed crucial infrastructure, leaving the Kastrians to eke out a miserable post-apocalyptic existence. They collectively decided to die with dignity instead, rather than give Eldrad the satisfaction of seeing them thus reduced. The king even leaves a mocking message for Eldrad, who he correctly predicts will not have considered this possibility.
- Fatal Family Photo: Fatal "last conversation with the wife and kids", anyway. Subverted when the Professor is saved at the last second rather than being killed off.
- From a Single Cell: Eldrad first appears as a fossilized hand, then having absorbed some nuclear radiation, turns into a walking hand. Eventually it regenerates into an entire person.
- Ghost Planet: Kastria.
- Glowing Eyes of Doom: Eldrad.
- Helping Hands
- Hypnotic EyesDoctor: "Sarah, I want you to look into my eyes."Sarah: "OH NO, NO THAT'S NOT F__" (falls into trance)
- I Love Nuclear Power: Eldrad can use radiation to regenerate a body! That's a useful trick.
- Insult Friendly Fire: When Eldrad explains he chose his first form to match the 'primitives', the Doctor remarks that Sarah was the first 'primitive' Eldrad met. Sarah kindly reminds him she doesn't like to be called primitive.
- I Was Just Joking: But you gotta go home anyway, Sarah.
- Large Ham: Eldrad, after returning to his true form, starts to devour the scenery. This seems to be a trait among most Kastrians — the few others we see do a lot of shouting.
- Meaningless Villain Victory: Eldrad returns to Kastria to become its ruler, only to find the planet entirely dead. A final message from King Rokon (the king who Eldrad planned to usurp) crowns him "King of Nothing".
- Mind Control: Eldrad's hand can control anyone who's come into contact with it.
- Most Annoying Sound: Eldrad does not like the sound of the nuclear power plant's alarm. Professor Watson is not crazy about them either.
- Nuclear Option: Eldrad's taking over the nuclear reactor is considered dangerous enough to nuke it.
- Put on a Bus: Sarah Jane, who returned in 1983 for "The Five Doctors".
- Rage Quit: Sarah finally decides she's had enough of all the crap she's been through and decides to leave the TARDIS. Pity she changes her mind at the worst possible time.
- Running Gag: Anyone describing Sarah Jane comments on how much her outfit resembles Andy Pandy's.
- A person asks where the Doctor is from. When told "Gallifrey", they ask "Is that in Ireland?" This comes up again on rare occasion during the classic series.
- Sequel Hook: The call from Gallifrey leads directly into the next episode.
- Silicon-Based Life: The Kastrian race.
- Stun Guns: Eldrad's ring can knock out people for a time ranging from a few seconds to about an hour.
- Talent Double: In the final scene, Sarah Jane whistles the tune "Daddy Wouldn't Buy Me a Bow Wow". Since Elisabeth Sladen couldn't whistle, director Lennie Mayne provided the whistling while Sladen mimed to it.
- Taught by Experience: Well-used to traveling in the TARDIS and not liking the idea of helping Eldrad, Sarah channels her inner Deadpan Snarker when it suddenly lurches in flight.Sarah: (pops into the control room, eating a banana) "Off course again, are we?"
- This Is Not a Drill
- Ultimate Defence of the Realm: The RAF try to destroy the nuclear power station and Eldrad with a nuke. It doesn't work.
- Women Are Wiser: A villainous example. Eldrad is in fact evil, but the female version is much more thoughtful and calculating, while the male version is a blustering moron who gets himself killed by tripping over a scarf.
- The X of Y