The Eighth Doctor wants to visit his old poetry buddy William Wordsworth, but the TARDIS instead takes them to Lucie's auntie Pat in the 1980's. They're a bit confused. Pat isn't exactly happy to see them, because last time they visited, people got eaten and stuff. Also, she wishes that the Doctor would stop calling her "auntie".
Meanwhile, two businessmen also arrive at the hotel that Pat runs together with her husband Trevor. They're in the music industry, and they're hoping that Trevor is up for Putting the Band Back Together one last time. He's not. In fact, he's horrified to see them. Lucie notes that Pat never mentioned a Trevor, and that her life right now seems to be significantly more interesting than that of the future Pat that she's known all her life.
Also, there's a giant lake monster. Lucie goes to investigate and sees the businessmen (and one woman) suckling on the monster's teats. She's very squicked out by it, and quickly gets noticed.
The Doctor quickly figures out that Trevor is, in fact, a body-snatching Zygon, with the lake monster being their pet Skarasen. The original human Trevor is stuck in a body-copying machine in the hotel basement. He and Lucie (who's acting a bit strange) go tell Pat. But Pat already knows, and doesn't even care that her husband is an alien. They're in love, they're having a grand time being eco-hippies and running a hotel, and they'd like to just be left alone now.
The Zygon businessmen conclude that "Trevor" is insane, and that he's deluded himself into believing he can feel a weird human emotion like "love". Trevor and Pat, however, really are happily in love. In fact, the human body in the basement isn't even alive anymore; it was dead when Trevor the Zygon found it, and the Trevor that Pat came to know and love was always just him. The Doctor apologises for having assumed the worst. However, he also realises that Lucie is Not Herself, and is in fact the female Zygon in disguise. Lucie's been kidnapped and is stuck in a body-copying machine somewhere. And the Zygons have come to take Pat's organically-fused Zygon necklace.
The Doctor figures out the Zygon plan: using global warming, they want to make Earth suitable as a new home planet. They engineered the rock music craze of the 1980's (hair spray, compact discs with invisible greenhouse gasses, etc. etc.) to hurry up that process. Which also explains Trevor's passionate love for the environment: he wanted to save the Earth instead. And so, Trevor makes a Heroic Sacrifice and destroys the ship. But he's too late to save Pat: blood was still gushing out of the wound from her ripped-off necklace, and the destruction of the ship cost her her life.
Lucie is woken up by a very frantic Doctor. She's missed the whole adventure, and is absolutely horrified to learn that her aunt is dead. The Doctor explains to her that since auntie Pat was pretty much a nobody in the future, time can adjust itself to just sort of... skip around her. That doesn't make Lucie any happier, and she wants to stay at the hotel for the remainder of the night to mourn Pat and make sure the customers are taken care of.
The Doctor goes outside to look at the pretty sky and recite poetry for a bit. And to call Trevor out of the bushes where he's rather obviously hiding. Trevor is devastated to have lost his wife, but he can honour her... by concentrating really hard, transforming himself into her forever, and taking on her form for the rest of a natural human lifespan without needing his Zygon body-copying machine. The Doctor eventually agrees to always lie to Lucie about it. What Could Possibly Go Wrong?
- Attack Animal: The Skarasen.
- Body Horror: Pat's necklace.
- Boomerang Bigot: Pat accuses the Doctor of being one when he makes a fuss about Trevor's species.
- Cultural Rebel: Trevor to the Zygons.
- Gender Bender
- Going Native
- Green Aesop: Played straight.
- Homoerotic Subtext: Mims seems awfully eager to share a bed with Urquhart.
- Human Popsicle: The original Trevor.
- Interspecies Romance: Pat and Trevor.
- Jumping Off the Slippery Slope: As we find out in later episodes, Lucie considers the Doctor's decision to lie to her Jumping Off the Slippery Slope, and it leads her to break off their friendship. She eventually forgives him when much worse things happen to both of them, but their friendship is never the same again.
- Minion with an F in Evil: Mims.
- New Powers as the Plot Demands: Zygons can permanently change form just by thinking really hard.
- Nice Job Breaking It, Hero!: If only Trevor hadn't given Pat the necklace to wear...
- Screaming at Squick: The Zygon interlopers discuss how normally, humans scream and sometimes faint upon seeing their true forms.
- Shout-Out: The title and cover art reference the 1976 film The Man Who Fell to Earth, another story of an alien masquerading as a human. (Trevor taking on the form of a human musician likely references the fact that the title role was played by David Bowie.)
- Stealth Pun: The Bygones Guest House. Probably not the first "bygone"-"Zygon" pun in Who, and sadly, probably not the last either.
- Sweet Tooth: The Doctor habitually takes his tea with six sugars...
- Your Favorite: ...so Lucie knows to make it that way for him.
- There Is Only One Bed: Discussed when Urquhart and Mims are checking into the guesthouse. Mims says he'd be cool with it.
- Un-Evil Laugh: Mims'. He sounds like he's trying much too hard to fake it.
- Was It All a Lie?: After meeting Zygon!Lucie, Pat asks the Doctor if her niece Lucie ever really existed, although she of course does.
- What Is This Thing You Call "Love"?: Almost literally said by the Zygons.
- You Called Me "X"; It Must Be Serious: The Doctor only remembers that you don't call people by family-related titles just 'cos your friends are related to them when Pat is dying.