A Slitheen

The Doctor has landed the TARDIS in Cardiff to use a rift in time to recharge the TARDIS. As the process will take a whole day The Doctor, Rose Tyler, and Captain Jack Harkness take the opportunity to explore the area. They are also joined by Rose’s boyfriend Mickey. But while having a meal in a restaurant The Doctor recognises a photo in a local newspaper of an enemy he thought long since dead – Margaret Blaine who is really a Slitheen in a human disguise.

Realising that the Slitheen must be up to no good - especially as she is posing as the Mayor of Cardiff - she must be stopped. The Doctor finds Margaret Blaine at the City Hall. On being discovered by The Doctor she tries to escape but after chasing her through repeated uses of a teleporter the Slitheen realises that it is not going to get away.

The Doctor then learns of the Slitheen‘s plan to build a nuclear power station in Cardiff City as a disguise for an alien plot to rip the world apart. She hopes that the meltdown of the plant would open the Rift and so destroy the planet. The Slitheen then plans to use a tribophysical waveform macro-kinetic extrapolator - a pan-dimensional surfboard - to escape the explosion.

Trusty Sonic Screwdriver

The Slitheen is forced to come quietly as The Doctor plans to take her back to her home planet of Raxacoricofallapatorius. But first they have to wait for the TARDIS to recharge its power supply and so while they wait The Doctor takes the Slitheen for a last meal. Despite several attempts by the Slitheen to kill The Doctor, through various means, The Doctor starts to sympathise with the Slitheen as it is revealed that by returning her to Raxacoricofallapatorius will mean instant death as the Slitheen family are convicted criminals. The Slitheen requests that The Doctor takes her to a different planet instead. However, before The Doctor makes his decision, a large earthquake shakes the area.

The Doctor discovers that not everything is as it seems and that the Slitheen may be able to escape. When he returns to the TARDIS he discovers a bright column of light is shooting up overhead. Captain Jack Harkness tells The Doctor that it is the power from the Rift, brought upon by trying to use the extrapolator to speed up the recharging of the TARDIS.

Margaret

As the rift in time starts to break open The Doctor realises that this was the Slitheen’s plan all along - that someone of advanced enough technology who would recognise the Slitheen, who would hopefully activate the device, causing it to lock onto the nearest alien power source thereby tearing open the Rift and eventually the Earth.

As the Slitheen attempts her escape the heart of TARDIS opens and she is consumed by the full force of the Rift. After The Doctor and Captain Jack Harkness manage to close the TARDIS console and so reseal the Rift once more they discover that the Slitheen has reverted back to an egg. The Doctor surmises as the TARDIS is telepathic, it may have sensed that the Slitheen wanted a second chance and so gave that to her. And so The Doctor, Rose, and Captain Jack Harkness prepare to travel to Raxacoricofallapatorius, leaving Mickey behind, so as to deliver the egg.