Boom Town ( TV story ) Boom Town was the eleventh episode of the first series of BBC Wales " 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 . Synopsis . The Ninth Doctor , and his companion s , Rose Tyler and Jack Harkness travel to modern - day Cardiff and meet up with Rose ' s boyfriend , Mickey . There , they discover that their " enemy " , Blon Fel - Fotch Passameer - Day Slitheen , is very much alive , if without an easy escape route from Earth , and is willing to rip apart the planet to ensure her freedom . Plot . In an office in Cardiff , a scientist brings his concerns to the Mayor Margret Blaine over a new nuclear power plant to be built there . It is dangerous , almost as if it had been intentionally built to explode . Blaine 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 Blaine 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 Blaine , whom they known as 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 , realising that she herself no longer has one . Realising 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 recognises 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 apologises . At the request of Blon , the Doctor joins her for one last meal at her favourite 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 , bringing up her childhood and her last - minute change of heart over killing Cathy . Though the Doctor dismisses her act of kindness as a way of living with herself , he does sympathise . Before he can agree to take her elsewhere , however , 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 . Blon reveals that this was her plan all along — the extrapolator would have been found by someone of sufficiently advanced technology to recognise 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 manages 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 realises that Mickey has left ; the Doctor offers to wait for him , but Rose lets him go , allowing him to also have a second chance .