Boom Town was the eleventh episode of the first series of the revived series of Doctor Who. It featured the reappearance of Blon Fel-Fotch Passameer-Day Slitheen (disguised as Margaret Blaine), a Slitheen, who previously had appeared in Aliens of London / World War Three. It also included significant development of the relationship between Rose and Mickey.
The episode was the first to be set in modern Cardiff, and established that the Cardiff Space-Time Rift, implied in The Unquiet Dead, was still present in the 21st century and releasing enough energy to fuel the TARDIS. It thus laid critical narrative groundwork for Torchwood’s central theme. It also introduced the extrapolator and established the energy present at the heart of the TARDIS — both crucial elements of the series 1 finale. Finally, it contained the main characters’ first major recognition of the Bad Wolf meme.
In an office in Cardiff, a scientist brings his concerns to the Mayor over a new nuclear power plant to be built there. It is dangerous, almost as if it had been intentionally built to explode. The Mayor asks him if he has told anyone else about his findings. He replies that he did not, and instead went directly to her. She commends him for making the right choice — as she apparently and audibly experiences some gas. As the scientist expresses his relief that the Mayor will shut down the project, she reveals herself to be a Slitheen, and kills the scientist.
The Ninth Doctor has landed the TARDIS over the Cardiff rift located in the Roald Dahl Plass, using slow radiation leakage to recharge the TARDIS. As the process will take a whole day, he, Rose, and Jack are joined by Mickey Smith and take the opportunity to explore the area. While they enjoy a meal at a restaurant, the Doctor notices to his dismay the front page of the Western Mail, with the headline “New Mayor, New Cardiff” and a picture of Margaret Blaine — the human form of Blon Fel-Fotch Passameer-Day Slitheen whom they previously had encountered. Since their meeting, Blon has become the Lord Mayor of Cardiff, and initiated the construction of a nuclear power plant. However, several people had found significant flaws in the design that could lead to a nuclear meltdown, and had approached her about these issues, but they have since disappeared, Blon having killed them herself. During a press conference, a young reporter named Cathy Salt approaches Blon about these deaths and the information they had left behind. Blon thinks she should have a word in private; a loud rumble in her belly gives her an excuse to go to the toilet and she takes Cathy with her. Disgusted as she hears Blon on the toilet, Cathy notes they got there just in time. Blon gets out of her skin suit and plans to kill her, but has a change of heart as Cathy talks about her family and unborn child, realizing that she herself no longer has one.
Realizing that they must stop Blon, the Doctor’s group converges on City Hall and eventually capture Blon after chasing her through repeated uses of a teleporter. She tells the group that the teleporter is how she escaped the destruction of the rest of her family, and that she hopes that, as planned, the meltdown of the plant would open the Rift and destroy the planet, with her using a hidden tribophysical waveform macro-kinetic extrapolator — a pan-dimensional surfboard — to escape the explosion. The Doctor notices that the name of the plant, Blaidd Drwg, is Welsh for “Bad Wolf”, a phrase that he has observed before in his adventures with Rose. The Doctor tells Blon he will take her back to her home planet of Raxacoricofallapatorius, but Blon notes that the Slitheen family are convicted criminals there and she will be executed, which the Doctor insists is not his problem.
Jack recognizes that the extrapolator can be used to halve the time to refuel the TARDIS, and stays there to install it. Rose and Mickey go out for a drink to discuss their relationship; Mickey, while inviting Rose to a hotel room, claims to be seeing someone else since Rose is not there for him. Rose counters that she knows the woman, that Mickey doesn’t even like her and that “that’s never gonna happen, so who do you think you’re kidding?” She argues that this conversation has nothing to do with Trisha. Mickey says that he can’t even go out with a girl from “the shop” because Rose picks up the phone to say she’s coming back to present-day Earth and Mickey comes running for her. When Mickey claims he’d wait for Rose for the rest of his life, Rose apologizes.
At the request of Blon, the Doctor joins her for one last meal at her favorite restaurant, equipped with bracelets that will electrocute Blon if she gets more than ten feet away from the Doctor. Blon attempts to kill the Doctor through various means, but the Doctor is able to casually block the attempts. Blon then attempts to gain the Doctor’s sympathy, explaining how she will be executed and enquiring if he could take her to a different planet instead. However, before the Doctor can agree, a large earthquake shakes the area.
The group reassembles in the TARDIS, where a bright column of light is shooting up overhead. Jack tells the Doctor that it is the power from the Rift, drawn by the extrapolator. The Doctor realizes that this was Blon’s plan all along — the extrapolator would have been found by someone of sufficiently advanced technology to recognize the Slitheen, and would have activated it, causing it to lock onto the nearest alien power source (the TARDIS in this case), to tear open the Rift and eventually the Earth, while she would have still ridden the device to escape the destruction. Blon takes Rose hostage, choking her, and demands the extrapolator, or Rose will die. The Doctor warns her that this isn’t just any ship her device has latched upon – this is the TARDIS. Before she can use the extrapolator, the heart of the TARDIS opens and shines in her face; Blon dreamily looks into the light with a smile, and then beams at the Doctor, emphatically telling him, “Thank you.” The light overtakes her, and shortly her skin suit falls empty to the console floor. The Doctor and Jack manage to close the TARDIS console and reseal the Rift once more. When they investigate the suit, they find a Slitheen egg; the Doctor surmises as the TARDIS is telepathic, it may have sensed that Blon wanted a second chance and gave that to her. As the Doctor, Rose, and Jack prepare to travel to Raxacoricofallapatorius to deliver the egg, Rose realizes that Mickey has left; the Doctor offers to wait for him, but Rose lets him go, allowing him to also have a second chance.