The one with an evil cactus.
On the planet Tigella, life revolves around the Dodecahedron, a mysterious alien artifact which powers Tigellan civilization. Some Tigellans worship it as a gift from their god, while others consider it just an object to be scientifically studied. With tensions between the two groups running high, the Tigellan leader sends a message to the Doctor, who has visited Tigella once before, inviting him to come and arbitrate between them.
The message is intercepted by Meglos, the last survivor of the neighbouring planet Zolfa-Thura; the scientifically-advanced Zolfa-Thurans created the Dodecahedron shortly before wiping themselves out in a massive war. Meglos uses his technology to trap the TARDIS in a time loop and then to disguise himself as a double of the Doctor, and travels to Tigella himself. He manages to steal the Dodecahedron and is about to leave when the real Doctor, having escaped the time loop, arrives and is promptly accused of the theft. There is much confusion, and although it becomes clear that there are two Doctors roaming about, the fake Doctor manages to get away with the Dodecahedron. Meglos returns to Zolfa-Thura, where he plans to use the Dodecahedron to power a superweapon capable of destroying entire planets.
The Doctor and Romana follow, and the Doctor attempts to turn his trick against him by telling Meglos's hired henchmen that he is Meglos and that any other Doctor they see about the place is the real Doctor and should be locked up. This works long enough for him to mess around with Meglos's superweapon, and when it is fired it explodes, taking Meglos, his henchmen, and the planet of Zolfa-Thura with it.
Oh, and the Dodecahedron, leaving Tigellans of all ideological persuasions to work together to rebuild their civilization using some other power source.
- Bad Bad Acting: The solution to the time loop.
- Chekhov's Gun: Romana is attacked by killer plants. Not only does this separate her from the Doctor for most of the story, she leads Meglos' henchmen back to the plants, and uses them attacking as a distration to escape.
- Chronic Backstabbing Disorder: Meglos anticipates his allies betrayal, and establishes the ground rules straight off.
- Fighting from the Inside: Meglos kidnaps a mild-mannered, terrified human from 20th century Earth and possesses his body. Unfortunately for Meglos he proves to be tougher than he looks.
- "Groundhog Day" Loop: Defied. The two Time Lords solve a time loop within 5 minutes before starting the adventure proper. But then, they are Time Lords, it should be easy for them.
- Heroic Bystander: George Morris (aka The Earthling) proves that despite his geekiness and Nerd Glasses, that he's just a pinch Too Spicy for Yog-Sothoth.
- Human Sacrifice: The Doctor is offered up to Ty almost.
- Impersonating the Evil Twin: The Doctor impersonates Meglos. He is discovered only when Meglos is shot in the belief that he's the Doctor and replies, "satisfied?"
- Kick the Dog: Literally when the mercenary kicks K-9. According to the commentary, the actor, who was best known for playing hammy villains, added this business himself because he figured the fans would be expecting it.
- Leitmotif: The titular angry cactus is signalled by a vibraslap scream, with increasing distortion as he approaches Villainous Breakdown.
- Man-Eating Plant: The plants that attack Romana and later, Meglos' henchmen.
- Names to Run Away from Really Fast: Meglos
- Our Monsters Are Weird: Meglos. A dangerous and psychotic alien Diabolical Mastermind, technocrat and shapeshifter, who accomplished all this despite being a sessile cactus with no discernable sensory organs.
- Plant Aliens: The titular Meglos, an evil shape-shifting cactus who wanted to take over the universe. What a prick.
- Playing Possum: One of the pirates.
- Protagonist-Centered Morality: The Doctor's decision to kill Meglos and the band of thieves he was working with (who didn't even know they were involved in a genocidal scheme up until the end) is never explained or even questioned. More alarmingly, the underground city leaders thank the Doctor for saving their planet from destruction and stopping the criminal who stole their power source, but never question whether it was really necessary to destroy the power source. They now have to evacuate untold cubic miles of powerless underground dwellings, set up new homes in a horrible writhing jungle, and somehow find the power to make it all work. Assuming this is even possible, it will be a long and difficult process in which many lives will no doubt be lost. The Doctor may have had no alternative to destroying the artefact, but we wouldn't know because the subject is never discussed at all.
- Recycled Soundtrack: The sound effect created for the approach of the Fendahl is re-used as background atmosphere for the Tigellan jungle, as well as effects used in The Daleks' Master Plan and Planet of the Daleks.
- Significant Anagram: Brotadac is an anagram of "Bad Actor".
- Single-Biome Planet: Tigella is a jungle planet. Zolfa-Thura is a desert planet (although it has the excuse of having suffered a life-extinguishing catastrophe).
- Terminally Dependent Society: The people of Tigella live in a city where everything is powered by a single alien artifact, which gets destroyed at the story's climax; although they're initially horrified by having to manage without it, it's presented as ultimately being an opportunity rather than a disaster.