The Fifth Doctor, travelling alone since Peri and Erimem are otherwise occupied, notices strange energy spikes on Earth and visits the Gogglebox. He meets a young fan-geekoid named Alan, who's just started working there, and decides to see what's causing the disruption, accidentally leaving behind a book in the process. (Alan refuses to travel with him because there's no record of him being a TARDIS traveller.) When he realises that his future self is already investigating 1984 Baltimore, he heads off to the next spike instead: Brisbane, 2006.
Meanwhile, Tegan, now 46 years old, is trying to avoid celebrating her birthday. Her friends throw her a surprise party anyway, at a high-end club called the 8687. The Doctor soon gets mixed up in the whole mess: the energy signal leads him to the office of Dr. Kathy Chambers, formerly Peri's best friend and now a naturalised Australian who's horrified to suddenly have the Doctor in her life again. Her friend James, an Australian tech whiz, helped her and Nate emigrate to Brisbane. She's still trying to cope with losing her father and brother (who didn't make it) to the Cybermen all those years ago, and simply wants to run her office in peace, with a bit of pharmaceutical research on the side. Also, Tegan's her patient, which the Doctor finds out when he charms his way into the 8687 and accidentally ends up at the party.
While Kathy realises that this is a younger version of the Doctor, and tries to figure out why he didn't recognise her back in Baltimore all those years ago, Tegan force-feeds the Doctor his own lapel celery and tells him to act. normal. and not to kill anyone. Five is pretty distraught to find out that Tegan isn't a glamorous travel agent or something, but simply took over her father's animal feed company. Everyone who's present for her party is either from her office or, in Kathy's case, someone she knows out of necessity. Tegan glumly reveals the reason she's not in the mood to humour Five: she's dying of cancer and has about two months to live.
The Doctor tries to recover from the shock, and begs Tegan to let him find a cure for her. She won't have it — she'd rather spend two months just living her normal life than risk her loved ones being harmed by the Doctor's presence. The whole thing gets rather rudely interrupted when Tegan is kidnapped at gunpoint by Kathy and her friend James. The Doctor follows them, together with Tegan's ex (and co-worker) Mike and Mike's new girlfriend (and co-worker) Jodi. Tegan vehemently hates both of them, but the pair cares enough to make an attempt at rescuing them.
Jodi, who's got plenty of Hidden Depths, manages to blackmail her way into Kathy's office and is held at gunpoint by James while she surreptitiously keeps her cellphone on. Kathy soon reveals the reason she needed Tegan: she's got Nate in the back of her office. Half-converted into a Cyberman, using the technology left behind by the Doctor in Baltimore. And she blames herself for getting Peri's mother, Janine, killed all those years ago by messing with the Cyber-technology. Since she's absolutely sure that the police would eventually have blamed her, she's eternally grateful to James for getting her and Nate to Australia. So grateful, in fact, that she goes along with his plan to shut down the entire phone network of Brisbane... and to use the ensuing panic to cyber-convert all of humanity. Just out of sheer kindness.
The Doctor realises that Kathy is completely mental, and that's it'll be his fault in the future. He causes a distraction by karate-chopping James' van and the group infiltrates into the building. Sadly, Jodi dies in the process, and Mike realises just why Tegan never mentioned the Doctor during their relationship — if this is the sort of thing that usually happens around him, Tegan would of course do anything to avoid him for the rest of her life. And Mike has another epiphany: he realises that Tegan was in love with the Doctor all along.
Tegan's brain tumor turns out to be the key to the cyber-conversion, since it's not simply cancer, but an alien something she picked up during her travels. Her alien bits can provide very useful information to James' pet project: a medical A.I. called System. But now that James has the Doctor, he can become part of System instead. Locked into the computer, the Doctor quickly befriends System, installs a failsafe and makes it give him a hypnotic reminder — "8687" of what happened this day. Since he won't recognise Kathy in her past and his future, he assumes that he'll suffer amnesia. System will enable him to telepathically broadcast a random reminder, "8687", to people around him, making them say it out loud until he's triggered into remembering Kathy's future when he meets her in Baltimore. However, James (predictably) soon betrays Kathy and runs off with a copy of System, planning to start a project of his own.
The Doctor talks Nate, who's barely conscious, into committing suicide and ends up blowing up the entire doctor's office. Kathy is given a second chance at a new life, since she's assumed to have died in the explosion. Tegan and Mike decide to spend the rest of her short life together — although she admits she did love TARDIS travel after all, she'd rather lead a short, normal life than take all the risks that come with finding a cure for whatever alien affliction she has. She also laughs right in Five's face when he bashfully brings up the idea that she might have been in love with him. After a sweet hug, they say their final goodbyes.
- Actor Allusion: Alan, the ultimate Doctor Who fanboy, is played by Nicholas Briggs'. He says there's no record of him ever travelling with the Doctor. Presumably, no official canon record...
- Arc Number: 8687, continued from the previous episode. Also counts as Leaning on the Fourth Wall — it's the release numbers of the episodes.
- Borrowed Catchphrase: Tegan tells Five to have a "Brave heart" at their goodbye.
- Brick Joke: A reverse one with the book from the previous episode.
- Call-Forward: The adventure starts off with an advert for the new Miami Vice movie starring Collin Farell, the TV series gets a mention in the second part of this trilogy, the Reaping. He also forgets his book.
- Computer Voice: System shows up again, in her first appearance.
- Continuity Porn: "The Visitation" and "Earthshock" are referenced. The Cybermen plot from "Spare Parts", "The Harvest" and "The Reaping" is all neatly tied together in a very Timey-Wimey Ball way. Alan, who's a stand-in for the audience, references a lot of Doctor Who trivia, ranging from direct references to the great fire of London and the Mary Celeste to the invention of fire. And, for the entirety of the episode, Peri and Erimem are on a mission in 1966 Monte Carlo.
- Continuity Snarl: The Doctor's infamous Time with UNIT is brought up, and the Doctor mentions Alan shouldn't ask when he did.
- Dragon with an Agenda
- Drives Like Crazy
- Dying as Yourself: Tegan.
- Have We Met Yet?: For the Doctor, this adventure happens in reverse. Seven started it off, Six had the middle part and Five, the earliest of the three incarnations, finishes it up.
- I'm a Doctor, Not a Placeholder: I'm a Doctor, not a killer.
- In Vino Veritas: Tegan told Mike about the Doctor once when she was really drunk. He didn't believe much of it.
- Jade-Colored Glasses: After leaving team TARDIS, Tegan became disillusioned with life on Earth, and never even went on holidays — nothing could compare to space.
- Jumping Off the Slippery Slope: The Doctor warns against this.
- The Lad-ette: Tegan hasn't changed a bit; when someone tries to make an alibi for her having gone shopping, nobody buys it.
- Lampshaded Double Entendre: Several with Jodi.
- Never the Selves Shall Meet: The Doctor wants to go to Baltimore first but hears a newscast from that time period how he's already on the lam, so he decides against it.
- Nice Job Breaking It, Hero!: The Doctor is the cause of Tegan's love life, apparently.
- Precision F-Strike: Tegan drops a BITCH quite harshly.
- Real Life Writes the Plot: Tegan is implied to have died of cancer in the story, because Janet Fielding was insistent at the time that she would perform in only one Doctor Who story, and this was their way of sending the character off with closure. Thank Rassilon for the Timey-Wimey Ball, because she changed her mind later on. Also became a bit Harsher in Hindsight when, right around the time she decided to come back the second time, Janet Fielding revealed she was fighting cancer. Luckily she recovered.
- Reality Ensues: Much like in the previous episode.
- Running Gag: The Doctor has a hard time getting Dr. Chambers to speak to him.
- Throughout Big Finish, Five has a habit of getting into villains' lairs by simply knocking on the front door (as in "Phantasmagoria" and "Singularity"). It's alluded to here, but the Doctor decides against it.
- The Scream
- Tap on the Head: The Doctor gets taken out with one.
- The "The" Title
- Timey-Wimey Ball: This is the last episode of a trilogy, in reverse order for the Doctor, who then manages to resolve the ends of the previous episodes retroactively, with a trigger phrase. Despite the fact this is the last part of the timeline for the Cybermen / Cybertechnology, but the first one for the Doctor.
- However, the story ends with something of a plot hole; Five still remembering the entirety of this adventure, and in The Reaping, 8687 didn't cause Six to recover anything past a vague feeling that Kathy is important. As of yet, there has been no continuation of this theme.
- Unwilling Roboticisation: Poor, poor Nate.