The one where Colin Baker shows up early.
Traveling alone together (with none of that, thank you), the Doctor and Nyssa are interrupted in their travels by a random alien force that tries to take over the body of the Doctor. The Doctor is perturbed, sure, but the Time Lords are completely freaked out by this. Worried that it might happen again, which would be bad news as the alien is made of anti-matter, the Time Lords decide to drag the Doctor back to Gallifrey and kill him. With a token show of regret, Lord President Borusa gets Colin Baker to shoot the Doctor and haul him off to the Disintegration Chamber. But someone has conveniently rigged the circuit so that he ends up just being shoved into the Matrix (no, not that one) where the anti-matter alien torments the Doctor... revealing that he's the long-lost (and allegedly dead) Time Lord Omega.
Meanwhile, in Amsterdam, a pair of English teenagers are menaced by a humanoid chicken with a label-gun. ...stop laughing! One of them is brainwashed, and he happens to be Tegan's favourite cousin, and Tegan happens to have lost her job, and happens to be visiting her cousin on this exact day. Then they all happen to be abducted by Omega, who wasn't actually aware that Tegan knows the Doctor at all.
As it turns out, the Doctor isn't dead. Omega brings the Doctor back and starts making a copy of the Doctor's body for himself. So the Doctor, Nyssa and the TARDIS run down to Amsterdam, to try and stop him, but they're too late. Omega sheds his massive helmet to reveal... the Doctor! In a most surreal scene, the Doctor and Omega-in-the-Doctor's-body banter back and forth before the Doctor reveals the big problem with Omega's plan: the body is only temporary.
Omega then leads the Doctor on a merry chase throughout Amsterdam, though he does take time out of his busy schedule to watch a Dutch street organ and test out his Villainous Creepy Smile on a little kid. The Doctor and Nyssa try to track him down, Peter Davison tries to speak Dutch and gets three out of three words wrong, and they freak out a lot of people around the Amsterdam grachten. Alas, Omega's antimatter body is dissolving, shown to truly gross effect by its face and hands dissolving into paste and green Rice Krispies. Eventually, Omega degrades to the point that he's a completely different actor, and the Doctor calmly (if reluctantly) takes out that label-gun and shoots Omega. Omega then fades to dust, and everything is back to normal.
...oh, and Tegan joins them again. The Doctor tries to put on a happy face, but it looks like he'd rather have Adric back.
This serial is notable in three ways. The first is that this features the return of Omega, who last appeared in the 10th anniversary special. The second is that this serial kicks off the 20th anniversary season of Doctor Who. The final? Sixth Doctor Colin Baker shows up for the first time — not as the Doctor, but as Jerkass security chief Maxil, the only Doctor until Peter Capaldi to appear in the series before landing the big role. In fact, there's even a scene of Maxil shooting the Doctor, something that's caused a lot of jokes within the fanbase and cast. Colin Baker himself joked that he'd gotten the part of the Doctor by shooting the incumbent.
- Alas, Poor Villain: Gareth Roberts once gave an interview for Doctor Who Magazine in 2011 in which he claimed there is no serial from the classic series that couldn't be saved by a good rewrite, using this as his litmus test: he suggested that even this story could be fixed by Russell T Davies, and that the audience would be crying for Omega at the end. Roberts tends to single out this story for every particular criticism of the Nathan-Turner era, suggesting that it's his least favorite ever.
- Anti-Villain: Omega, when he has the Doctor's body... just goes around enjoying having a body again and smiling at people. Okay the smile is creepy, but the guy's only company for the past million years have been monsters made of red bubble wrap in an anti-matter dimension... he's a bit out of practice with social interaction.
- Beware the Nice Ones: Nyssa shoots an awful lot of people in this episode, and even pulls a gun on the Lord President Borusa.
- Body Horror: The slow degeneration of Omega's new body.
- Combat Pragmatist: Maxil.The Doctor: Hello, I'm the Doctor.
Maxil: [shoots him with a laser gun]
- Continuity Nod:
- The Doctor talks about the Cybermen damaging the TARDIS console with laser fire, though no mention is made of this being a major contributor to his inability to rescue Adric.
- The fact that the TARDIS is supposed to be a Zone of Temporal Grace (where guns can't be used) is also referenced.
- Thalia comments on the Doctor's inability to return Romana to Gallifrey. The Doctor tells her that she chose to stay in E-Space.
- The dress worn by Nyssa at Cranleigh Hall is seen in her room.
- The Doctor asks Damon about Leela. He replies that she is well. The Doctor is sorry to have missed her wedding.
- Contrived Coincidence: Tegan being in the plot at all.
- Creator Cameo: John Nathan-Turner, wearing a sheepskin jacket, appears on-screen from behind the telephone box in part four. He would later claim this was a deliberate example of a Hitchcockian cameo, in reality he was trying to persuade passers-by not to get into shot.
- Extra Eyes: The Ergon appears to have four eyes on its face.
- Forgot About His Powers: Once again Time Lords have forgotten they have the ability to regenerate and save themselves from dying permanently. Extra odd when regeneration is actually mentioned by some of the characters.
- Friendly Target: Omega takes control of Tegan's cousin Colin and forces him to work (making him look pretty sick at the end of the story). When he realizes Tegan is a former companion to the Doctor, he threatens to kill her to make sure Colin cooperates (she was able to give the coordinates to where Omega was though).
- Ham-to-Ham Combat: Omega and the Doctor lob large roasts back and forth with glee as they devour everything in sight (not even Amsterdam is spared), even while Peter Davison plays off himself. Colin Baker does his absolute best to join in the fun.
- Hero of Another Story: Colin Baker played Maxil not as a guest role, but as someone who thinks he's starring in his own series.
- It's All About Me: Maxil is positively bored with the Doctor, and spends much of his screentime looking mildly offended or checking his nails. John Nathan-Turner actually asked Colin Baker to tone it down, because the show was called Doctor Who and not Commander Maxil. Baker replied that no person would ever consider themselves a minor character in someone else's life, and so he played Maxil as someone convinced he was the main character of the story. Of course, Baker soon would become the main character, making the whole thing quite Hilarious in Hindsight.
- Just Following Orders: Maxil.
- Mirror Match: A lovely scene where Peter Davison plays the Doctor and Omega — with the latter dubbed by Ian Collier, of course. This was so well done that many on the DVD commentary remarked that the voice fit Davison better than his own!
- Nice Job Fixing It, Villain!
- Not Even Bothering with the Accent: Marvel as Peter Davison struggles to pronounce "Jeugdhotel Centraal".
- Not Quite Dead
- Percussive Maintenance
- Reverse the Polarity: Nyssa claims it's the way to turn matter into antimatter.
- Scenery Porn: About a third of this adventure is in Amsterdam. Literally, this was the reason for the script being written. The script is able to Hand Wave it away, luckily, but it still feels damned strange.
- Sealed Evil in a Can: Omega.
- Sequel Episode: To "The Three Doctors".
- Shout-Out: The scene of Omega at the Punch and Judy show and interacting with a child was inspired by Frankenstein (1931) and Bride of Frankenstein.
- Shown Their Work: Everything about Amsterdam checks out: the scenery, the street organ, the clothes. The background music is indeed a very popular Dutch tune about Amsterdam. The Dutch people are played by Dutch actors using their regular Dutch accents. And although the city is actually two meters above sea level, its cellars are indeed a bit below. The only thing that's off is Peter Davison's truly atrocious pronunciation of Dutch, but luckily, he only gets three words ("Jeugdhotel", "Centraal" and "Frankendael"). Although the city's layout is all over the place, with Frankendael somewhere in the grachtengordel, and the chase in episode 4 teleporting between various parts of the centre.
- Unusually Uninteresting Sight: Averted. Watch when Peter Davison in monster makeup is staggering through Amsterdam in part 4: most of the citizenry barely notice, but there's a pair of people in one shot who outright goggle at the bizarre sight. Davison, on the DVD commentary, speculates that they weren't extras, and that it must have made for a weird holiday experience.
- Villainous Friendship: Omega appears to be sincerely saddened and angered by Hedin's death.
- Woobie, Destroyer of Worlds: Poor, screwed up, crazy Omega.... His confusion when running around in Amsterdam, while wearing the Doctor's body, is nothing short of adorable. The Third Doctor even feels upset about destroying him, saying he gave him the only freedom he could.
- The X of Y