Professor Rumford: Are you from outer space?
The Doctor: No, I'm more from what you'd call "inner time".
The search for the Key To Time takes the Doctor, Romana and K-9 to present-day (well, 1970s) Earth, where the Tracer leads them to a group of standing stones on Boscome Moor. The third segment is nowhere to be found, and the travellers instead meet the elderly Professor Amelia Rumford and her live-in assistant
Vivien Fay, who tell them that the site has had a varying number of stones over the years.
Then the inevitable campy druids show up and nearly sacrifice the Doctor, before it is revealed that the stones themselves are sentient — a lifeform called Ogri who feed on blood. One of the Ogri kills the head druid and it is revealed that Vivien Fay is in fact the Cailleach, a being worshipped by the druids, having taken on human form after being on Earth for four thousand years. Fay transports Romana to a spaceship hidden in hyperspace at the circle, and the Doctor follows.
Accidentally releasing two justice robots, the Doctor is sentenced to death. He takes a lawyer wig out of his pocket and instead begins a magnificent bit of Bothering by the Book
. He manages to knock Fay unconscious by poking her necklace, which legally allows the sparkly justice robots (called Megara) to read her mind, seeing as she can't speak for herself anymore. The Megara quickly notice that Fay is in fact Cessair of Diplos, the criminal they've actually been sent to try. She stole a few Ogri from a nearby planet and employed them on earth as her bodyguards while avoiding the interstellar police for four thousand years. They transform her into the final stone in the circle, but not before the Doctor has snagged her necklace — the segment of the Key that he has been looking for.
- Agony of the Feet: Romana's addiction to style results in a terrible choice of footwear at the start of this story- stiletto heels in a place loaded with stones. The Doctor calls her out on this several times, and they start giving her much discomfort. Naturally, Romana rids herself of them and goes barefooted by the end of the first episode when the shoes prove too much for her. They become Chekov's Gun when the Doctor discovers them discarded in a field. At the Doctor's insistence, she switches to more comfy and practical boots. But changes her whole outfit to match them.
- A.I. Is a Crapshoot: The Doctor's tale of justice machines.
- All Crimes Are Equal: Not exactly equal, but death is probably the most merciful, and definitely the most common sentence the Megara use.
- All Myths Are True
- Amoral Attorney: Even the Megara that's supposed to defend the Doctor pleads the death penalty.
- Ancient Astronauts
- And I Must Scream: Cessair's fate.
- Art Shift: Fourth and final Tom Baker episode shot entirely on video. The show would ditch film for good at the start of "Trial of a Timelord".
- Beat Them at Their Own Game: How the Doctor outwits the Megara.
- Bothering by the Book: Four once again shows his expertise at this.
- Call Back: The spaceship has a dead Wirrn in it.
- Celtic Mythology
- Cool Old Lady: Professor Amelia Rumford. When going out to find the Doctor, she takes a policeman's truncheon with her.
Romana: What's that?
Vivien: A policeman's truncheon. Last year, when she lectured in New York, she took it with her in case she got mugged.
Romana: And did she get mugged?
Vivien: No, she got arrested for carrying an offensive weapon.
- Cuteness Proximity: The Doctor's insistence on treating K-9 like an actual dog is quickly taken up by Amelia as well.
- Everything's Better With Sparkles: The Megara are pretty much just sentient sparkles.
- Exposition of Immortality: There are three portraits missing from the wall at Boscombe Hall. The Doctor and Professor Rumford come across them later, revealing them to all be portraits of Vivien Fay painted at various points during her long existence on Earth.
- Failed a Spot Check: Everyone gets ambushed several times over by eight foot tall moving rocks.
- Golem: The Ogri are a mix of this and Living Statue, since they're part of a stone circle.
- Halfway Plot Switch: In which a story about Druids and human sacrifice in a stone circle suddenly turns into a courtroom drama aboard a hyperspace ship.
- Homoerotic Subtext: Earlier drafts of the script made it much more clear how Amelia Rumford and Vivian Fay spent their time in that cabin. Somewhat astonishingly, Mary Tamm (Romana) didn't notice until fans pointed it out to her, and her comment was "we were all so innocent back then..."
- Human Sacrifice: Well, the druids didn't know the Doctor wasn't human.
- I Have Many Names: Cessair, aka Vivien Fay, aka Morgana Le Fay, aka The Goddess, aka The Cailleach... need we go on?
- I'm a Doctor, Not a Placeholder: Professor Rumford is an archaeologist, not an engineer.
- Judge, Jury, and Executioner: The Megara.
- Literal Cliffhanger
- Milestone Celebration: Averted. This is the 100th Doctor Who serial and its original broadcast was close to the series' 15th anniversary. There would have been a scene in which the Doctor would have gotten a cake and a new scarf for his birthday, but the idea was scrapped because it was considered to be too self-indulgent. (Although the cake had already been ordered, so cast and crew still got to tuck in).
- Mood Whiplash: The serial goes from a dark, gruesome horror story to... a black comedy about malfunctioning justice robots. Could be justified in that the show's had far too many tonal shifts for just one tone to be represented by the 100th story.
- Our Ogres Are Hungrier: The Doctor suggests that Gog, Magog, and Ogre could be inspired by the Ogri, large rocks that feed on blood.
- Plot Allergy: Cessair, like all Diplosians, has an aversion to citric acid, which helps in discovering her true identity.
- Really 700 Years Old: Vivien Fay has been on earth for 4000 years.
- Sealed Evil in a Can: A slight variation happened, where the Doctor releases a pair of biomechanical judges called the Megara from a ship stranded in hyperspace, and the judges promptly sentence him to death for letting them free without the proper legal authorization.
- Silicon-Based Life: The Ogri.
- Stripped to the Bone: The Ogri can do this to anyone who touches them.
- Techno Babble: The Doctor babbles quite a lot, but Professor Rumford pulls off a lot of Archaeo Babble too.
- Too Dumb to Live: When confronted with a door which is clearly marked as 'do not open', what does the Doctor do? Open it, of course, and get sentenced to death.
- The X of Y