Rose requests a trip back to the day her father, Pete Tyler, died. Reluctantly, the Doctor agrees, but he realises he has made a mistake, when Rose saves Pete from being run over by a car. This has now changed the timeline, and Reapers are transposing themselves all over the Universe. However, this time, the Doctor doesn't have a plan…

After chasing an unexplained capsule through time, the TARDIS lands in London, during an air raid. Rose investigates when a small child wearing a gasmask, standing on a rooftop, continuously calls out for his mummy. After floating above the city on a barrage balloon, Rose is swept off her feet by another time traveller – Captain Jack Harkness. Meanwhile, The Doctor follows a teenaged girl to a meeting of the homeless children of the Blitz. Here, the child in the gasmask is terrorising them with a parade of strange events, constantly asking for his mother.

After narrowly escaping the gas mask monsters, The Doctor, Rose, and Captain Jack try to stop the mutated people from infecting other victims. But it soon becomes apparent that one of the group may indirectly responsible for the virus. The answer lies at the bomb site, but time is running out...

The Doctor, Rose and Captain Jack stop off in present-day Cardiff to recharge the TARDIS on the Rift they formerly encountered in 1869. Whilst there, they come upon a since-thought-long-dead enemy of theirs, who is hatching yet another plan to destroy Earth.

In a seemingly futuristic world, the Doctor wakes up in the Big Brother house during an eviction, Rose wakes up on the Weakest Link with the Anne-Droid, and Captain Jack gets a make over with Trin-E and Zu-Zana. But something unusual is happening, and the losing contestants are being disintegrated. Obviously, it's time for the Doctor to investigate. Who or what is behind this?

The fate of the Universe is hanging in the balance as the Dalek fleet begin their invasion of Earth. The Doctor must decide whether to sacrifice every human being on Earth in order to save every other living creature in the universe from the Daleks. Either way, he knows he must send Rose home...