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The third story in the Cybermen arc, following "The Harvest" and "The Reaping".
The Fifth Doctor 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. Sh 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 very sweet hug, they say their final goodbyes.To Be Continued.