The Doctor and Mel take a shuttle from Io to Cressida, a former moon of Uranus that has been turned into a spaceship. It has been programmed to make adjustments to the Hub, the communications array located at Uranus. It is also the prime location to watch the collision of the asteroid Maleficent with the surface of Uranus. The Doctor is here to ensure this collision takes place, because in a few billion years, a new life form will be created, partly caused by the events of this collision.
On Cressida, Mel talks to two Pakhars while the Doctor chats up an old friend, newsreader Prentis Duke. Mel is then accosted by a famous film star named Mercy, who recognises that Mel and the Doctor are on Cressida illegally but seems to enjoy Mel's company.
Meanwhile, on Mars, Branko Chen and his Technix monitor Maleficent's path. His agents will sabotage the asteroid, causing a disruption in the communications system, which will allow Chen and his men to take over the Earth Federation.
When the sabotage takes effect, the Doctor heads to the engine room to effect repairs. Mel, meanwhile, realises that Mercy doesn't intend for the collision to take place. Mercy pulls a gun on Mel, but Mel escapes. She is then accosted by Prentis, who also pulls a gun on her, but his plan is different: he is dying, and he plans to make one last broadcast, exposing the corruption of the Earth Federation government. Mel agrees to help him.
Cressida collides with Maleficent, gently, and Chen's agents plan to use Cressida to nudge Maleficent off its intended path.
Mercy tries to stop the Doctor, but he recognises she is actually Merculite, a being from another time who arrived here using a time ring. He releases tachyons, which send Merculite back to her own time.
Unfortunately, the Doctor cannot fix Cressida's engines, and to ensure Maleficent collides with Uranus, he plans to pilot it into Uranus himself. Mel, having helped Prentis make his broadcast, joins the Doctor and mentions that Prentis is dying. The Doctor convinces Prentis to pilot the ship himself, while the Doctor, Mel, and the rest of the passengers escape to the TARDIS.
As they watch the collision, the Doctor mentions to Mel that the two Pakhars were the agents of Chen who sabotaged the ship. After the passengers disembark the TARDIS, the Doctor makes two trips: one to the Kuiper Belt in the past to nudge Maleficent out and on its way to Uranus, and one to the far future, where the Doctor greets the new life form by name — Prentis.
- Ice Warriors, Pakhars, Alpha Centaurans and Guermantians are all on Cressida.
- Mel thinks Io looks like a cheese and tomato pizza.
- The Doctor has twice landed his TARDIS in the middle of Trafalgar Square just before New Year's Eve.
- The Doctor gave Mel The Look and Learn Book of Galactic Civilisations: AD 3000-5000 to read.
- The Doctor mentions the television shows I'm a Chelonian... Get me out of here! and As the Worlds Turn.
- Maleficent has been pushed out of the Kuiper Belt.
- Mel drinks white wine from the Moon.
- The Doctor reads a note in his Five Hundred Year Diary.
- This was author Craig Hinton's final contribution to the Doctor Who franchise before his death in 2006.