The one with talking rocks.
The TARDIS lands in a quarry - it's not a quarry posing as an alien planet, but an actual quarry for a change - as the TARDIS is back in modern-day England. 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 thirty years later, it was, in fact... Aberdeen. On the opposite end of the country.
- 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.
- Bond One-Liner: After Eldrad's Disney Villain Death, the Doctor quips that the law of gravity finally caught up with him.
- 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.
- Creator Provincialism: Part of the story takes place at Oldbury Nuclear Power Station in Bristol, the hometown of writers Bob Baker and Dave Martin aka the Bristol Boys.
- 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.
- Fashion Dissonance: Sarah Jane's iconic "Andy Pandy" dungarees - a style of dress adopted by women in Summer 1976 as a result of a freak heatwave and an invasion of vicious, biting ladybirds, requiring breezy but full-body coverage (really). In particular, note the drawstrings around Sarah's ankles, keeping the trouser legs sealed from insects that would have been able to get up bell-bottoms. The heat, the ladybirds and the fashion trend were all swept away by a sudden downpour of rain that swamped Britain just as the first episode of "The Masque of Mandragora" was being broadcast on television.
- 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
- Humans Are Warriors: When his plan to resurrect the Kastrian race is foiled, Eldrad says that he'll become ruler of the primitive but aggressive humans instead and use them for his army of galactic conquest.
- Hypnotic EyesDoctor: (looks into Sarah's eyes) Now listen, I want you to concentrate.
Sarah: Oh no, that's not fair, not agai— (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.
- Living Relic: Eldrad was exiled and executed millions of years ago. After his resurrection, he forces the Doctor to transport him to his home world. The Doctor does so, taking him to Kastria in the present day. Once there, Eldrad discovers that all life on the planet died out millennia ago.
- 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.
- Non-Human Non-Binary: Eldrad has woman- and man-like forms.
- The Nth Doctor: Eldrads regenerated form is initially played by Judith Paris, but when he returns to his original form, he is played by Stephen Thorne.
- Nuclear Option: Eldrad's taking over the nuclear reactor is considered dangerous enough to nuke it.
- Otherworldly and Sexually Ambiguous: Subverted. When the Kastrian Eldrad regenerates from a severed hand into a glittering, androgynous female alien with a queenly bearing and claims of god-level scientific achievements, leading the audience (and even the Doctor) to read her as this sort of character. After the Doctor goes out of his way to take her in the TARDIS back to her own planet, we find out that he merely took on a tough feminine form because his regeneration was modelled after Sarah Jane, the first human he saw - when he is injured and regenerates into the male body which is apparently normal for Kastrians, he becomes a dull, thuggish, hollering idiot that the Doctor easily outwits. Sarah even tells the Doctor, "I quite liked her, but I couldn't stand him."
- Properly Paranoid: Even though there's only a one in three million chance of even a particle of Eldrad surviving his execution, the Kastrian decide to destroy their race bank, rather than take the chance that Eldrad will return and use them as a Sealed Army in a Can to conquer the galaxy.
- 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 story.
- 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 travelling 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
- Trip Trap: Eldrad comes charging after the Doctor and Sarah Jane, fails to see a thick multicoloured scarf stretched across his path and stumbles into a Bottomless Pit.
- 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