Boom Town

Plot
The Doctor lands the TARDIS in Cardiff, using the energy of the Cardiff Rift to recharge the engines. Rose has Mickey bring her passport to the TARDIS, and together with the Doctor and Jack Harkness, they go to lunch. At lunch, the Doctor spots a newspaper article showing Margaret Blaine, a Slitheen, has become Cardiff's new mayor. The four track down and capture Margaret to find out what she is doing there after their previous encounter. The Doctor observes Blaine's scale model plans for a new nuclear power plant, but identifies that it is purposely flawed to cause a meltdown that would open the Cardiff Rift and destroy the Earth. He also discovers the model contains a functioning tribophysical waveform macro-kinetic extrapolator that Margaret would have used to flee the Earth. The Doctor decides to take Margaret back to her home planet of Raxacoricofallapatorious, but she reveals that she has received a death sentence there and will be executed upon returning. The Doctor agrees to her final request to accompany her to a dinner meal. Margaret makes several half-hearthed attempts to kill the Doctor but he easily avoids them. She then tries to play on the Doctor's sympathies, asking him to take her to another planet instead.

Back at the TARDIS, Jack begins integrating the extrapolator into the TARDIS to speed up the engine recharge. Rose and Mickey hang out together, and he admits to her that he is dating someone else because she is not there for him, and he begs her to promise him that she eventually will come back to him in the end, but before she can answer him, loud sounds like thunder start to reverberate through the area. Cardiff is soon struck by a large earthquake that seems to be coming from the Rift. The Doctor, Margaret, Rose and Jack regroup and find that the extrapolator was a trap meant to redirect the energy from the TARDIS into the Rift, rupturing it. Jack and the Doctor are unable to stop the energy transfer, and Margaret takes Rose hostage and demands the extrapolator. The heart of the TARDIS opens on the console, bathing Margaret in light. While she is captivated by the light, Jack and the Doctor close the rift and disable the extrapolator. As the console closes, they find that Margaret's human suit is empty except for a Slitheen egg. The Doctor surmises that the TARDIS sensed that Margaret wanted a second chance at life and gave it to her. The TARDIS crew decides to return the egg to Raxacoricofallapatorius so Margaret can be raised in a different family. Rose realises Mickey has left without saying goodbye, and runs out to look for him in the nearby area, but he is gone. Rose returns to the TARDIS, clearly distraught, and the Doctor offers to wait a little longer so she can search for Mickey, but she declines, saying that he deserves better.