The one with the methhead mascot beast. Also the one where Mary Tamm plays a whopping four separate characters.
The search for the Key to Time takes the Doctor and Romana to the planet Tara, where feudalism and chivalry have endured. They split up to search, and Romana finds the fourth segment a few minutes in — it's part of a statue. However, she's promptly Trapped by Mountain Lions and captured by the villainous Count Grendel of Gracht, who thinks she's an android double of Prince Reynart's betrothed, Strella. The actual double was created for Grendel by his Ignored Enamoured Underling, Lamia. This way, Grendel is trying to usurp the throne, and will succeed if Reynart doesn't show up for his coronation. Also, he's got Princess Strella locked up in his dungeon.
Meanwhile, the Doctor also gets captured, by Prince Reynart's men. He's taken off to the Prince's hunting lodge and made to repair the Prince's android double, which will be used as a decoy. Grendel's men kidnap the Prince, but the Doctor is able to modify the decoy android so it can attend the coronation in his place.
The Doctor soon realises that Grendel plans to have Romana (or Strella or the android or whoever looks the part) marry Reynart, just so Reynart can be killed that same day and Grendel can proceed to marry the "queen" and take over the country. Grendel is a little annoyed when the Doctor refuses a part in this (after stringing along Grendel with wide-eyed fake enthusiasm for a while). He commissions a Romana android to kill the Doctor, but K-9 sees her for what she is and saves the day, while Romana herself escapes. Lamia dies in the crossfire and ruins Grendel's plans of using androids, since only she had the technology.
Finally, the Doctor rows across the moat to Grendel's castle and swipes the Key segment just before the Prince's men attack the castle, leading to the Doctor and Grendel having an absolutely magnificent Sword Fight. It's ended by the latter diving into the moat and escaping.
- Accidental Murder: Madame Lamia is shot down by crossbow fire. She's the only one to die in the serial.
- Affably Evil: Count Grendel.
- All Girls Want Bad Boys: Lamia's yearning for Grendel.
- And Now You Must Marry Me: Grendel's plan B for claiming the throne involves a rapid succession of weddings and widowings.
- Apathetic Citizens: The peasants don't seem to care for the whole Succession Crisis.
- The Archimandrite gets the prize: Grendel explains basically his whole plan (he plans the king's wedding, his funeral and his wife's second wedding in less than a minute) and the Archimandrite doesn't seem to remark anything strange. (The novelisation explains that after a lifetime at court he's learned it's best not to take sides when the nobles are scheming.)
- As Long as It Sounds Foreign: "Gracht" is Dutch for "city canal" or "moat". The actors make a valiant attempt at pronouncing it, but only get the "a" and "t" right and noticeably struggle with the "ch" throughout the story. (The "g", "r" and "ch" should actually all be guttural/rolling.) As later heard in "Arc Of Infinity" and "Vincent And The Doctor", this started a time-honoured tradition of no Doctor Who actor ever being able to pronounce Dutch. Of course, since the story takes place on another planet altogether, this comes with an easy Hand Wave here.
- Bilingual Bonus / Visual Pun: Dutch speakers will note that Grendel escapes by jumping into the gracht (moat) of castle Gracht.
- Come Alone: "They always want you to go alone when you're walking into a trap. Have you noticed that?"
- Costume Porn: Romana's purple and green hat-coat-trouser ensemble, which is apparently fashionable on Tara. In reality, Mary Tamm herself designed this costume.
- Damsel in Distress: Princess Strella can't care for herself as well as Romana can.
- Doppelgänger Replacement Love Interest: played with, Grendel doesn't really care whether Reynart marries Strella, Romana or an android, as long as he can claim it's Strella, and kill both when it suits his plans.
- Energy Weapon: Taran swords and crossbows shoot them.
- Even Evil Has Loved Ones: Though he's not in love with her, Grendel really cares about Lamia. Mostly because she knows how to make androids, but there's somewhat more than this.
- Fantasy Metals: The Key Segment is made from a material harder than diamonds - Lamia broke two diamond drill bits studying it.
- Feudal Future: The planet Tara seems modelled on Medieval England.
- Flynning: Played With. The Doctor ends up in a duel with "electro-swords". At first he seems incompetent with the blade, merely parrying blows. However, it quickly becomes clear that this is a ruse, as he unleashes more and more skill until finally besting his opponent with ease.
- The Guards Must Be Crazy: Sick Prisoner version.
- Heroes Gone Fishing: The Doctor resolves to spend this story fishing, leaving the leg work to Romana.
- High-Dive Escape: Count Grendel.
- Honour Before Reason: Reynart's swordmaster refuses to attack Grendel while the latter is under a flag of truce, even though he knows Grendel will break the truce.
- Hopeless Suitor: Madame Lamia to Count Grendel, and he takes advantage of it to further his ambitions.
- Hostage Situation: Reynart and Romana are forced to marry (and be killed afterwards) to save the life of Strella.
- Hot Scientist: Lamia.
- Identical Stranger: Strella and Romana look exactly alike.
- Ignored Enamoured Underling: Lamia.
- Impossibly Cool Clothes: To quote the BBC's webpage for this story, "The Archimandrite's hat has to be seen to be believed."
- Magnificent Bastard: Count Grendel. It's even mentioned in-universe by another character.
- Medieval Stasis: An odd example. The more advanced technology is used by peasants while the nobles live mostly without it.
- The Oath-Breaker: While Grendel technically never gave an oath of fealty to Reynart (the oath-taking ceremony was postponed after the coronation due to a combination of a homicidal Romana android turning up and Reynart's android needing maintenance), but he later meets up with the heroes under flag of truce and promptly destroys the Reynart android and re-kidnaps the rescued Romana.
- Offered the Crown: Grendel's Plan A was to have Reynart miss the coronation for some reason and thus become ineligible for the crown, after which the public outside the palace would start demanding that he be crowned in Reynart's place (at the instigation of some of his men hiding in the crowd).
- Religious and Mythological Theme Naming: The villains, Grendel and Lamia, are named after mythological monsters.
- Names to Run Away from Really Fast: Count Grendel of Gracht.
- Nice Hat: But top honours for hats must go to the Archimandrite's headpiece, which is very colourful.
- Recycled IN SPACE!: The Prisoner of Zenda IN SPACE!
- Ridiculously Human Robots: Several of them.
- Scenery Porn: The very beautiful forests, that's not to forget the location work at Leeds Castle.
- Shackle Seat Trap: Romana lies down on a bed for examination by a doctor, and metal brackets pop up and encircle her body to hold her there.
- Shout-Out: Count Grendel was named after the anatagonist of Beowulf.
- Speak Now or Forever Hold Your Peace: The Doctor gets there a little too early, and has to ask if they've gotten to that part of the wedding yet.
- Stage Whisper: Grendel whispers "think of the princess" to Romana and Reynart, who are on less than an arm's length of the Archimandrite, but he doesn't appear to have heard anything.
- Stellar Name: Princess Strella.
- Succession Crisis
- Sword Fight: The Doctor vs. Count Grendel, a very long and completely magnificent battle. The Doctor keeps his scarf on.
- Textile Work Is Feminine: Strella is working on an embroidery frame in her cell, and she and Romana are happily discussing the work when the Doctor finally finds them. (They've already taken the liberty of rescuing themselves.)
- Throwing the Distraction
- Trap Is the Only Option: Lampshaded.
- Villain: Exit, Stage Left: After the Doctor and his allies storm Grendel's castle, the Count shouts "next time, I shall not be so lenient!" before jumping off a parapet into his moat and swimming away.
- Whole Plot Reference: The Prisoner of Zenda.
- Widowed at the Wedding: Grendel plans to have "Strella" become Reynart's widow and heiress so that he can claim the throne as her second husband... and promptly become a widower himself.
- The X of Y