The Doctor and Christina

Tracking a mysterious energy signal, The Doctor boards a London bus and so meets Lady Christina de Souza - a young thrill-seeking cat-burglar who has just stolen a gold chalice once belonging to King Athelstan from a nearby museum. She is on the bus so as to evade the police who are after her.

As the bus enters a road tunnel, with the police in hot pursuit, it is catapulted through a wormhole to the alien planet of San Helios that is located on the other side of the universe.

There The Doctor must join forces with the extraordinary Lady Christina so as to find a means to return, along with all the other passengers, back to Earth. They soon realise that they have three problems. Firstly San Helios seems to be just one enormous desert – and the sand has already got into the engine – rendering the bus completely immobile, secondly the bus was damaged while it travelled in the wormhole, making it very dangerous for anyone to return through the wormhole unprotected – as proved when the bus driver tries enters the wormhole, only to exit as a skeleton.

Arrival

The third problem is though much more difficult for The Doctor to overcome – they are not alone on the planet – thousands of lifeforms are heading towards them that will not only kill everyone on the bus but also everyone on Earth if they are allowed to reach the wormhole.

The Doctor and Christina decide to scout the planet, while two of the passengers Nathan and Barclay try to fix the bus. The Doctor and Christina soon encounter stranded Tritovore traders and, when they find themselves on the damaged Tritovore’s ship, they discover that San Helios is meant to be a bustling world of billions with a thriving ecosystem. Soon, it becomes clear that the mysterious planet holds terrifying secrets and that the wormhole and the devastation of San Helios are linked. They also spot what appears to be an approaching storm – but which The Doctor realises is a large deadly swarm of metallic stingray-like aliens who routinely create wormholes and then destroy whole ecospheres. And this swarm is travelling very fast and with the wormhole still open, and growing in size, Earth is in grave danger.

The Wormhole

With the help of the Christina’s cat-burglar skills The Doctor is able to retrieve a crystal which powers the Tritovore’s spaceship (together with the claps used to fix it to the Tritovore’s spaceship) that he can use on the bus and so offer them a means of escape.

Unfortunately, in retrieving the power supply, Christina unintentionally awakens two stingrays, that have lain dormant within the Tritovore’s spaceship, The Doctor and Christina manage to escape back to the bus but the two Tritovore are not so fortunate.

Using the claps and the gold chalice of Athelstan, that Christina is forced to hand over, The Doctor is able to make the bus to fly through the wormhole, with the stingrays in hot pursuit.

Lady Christina de Souza

Meanwhile, back on Earth, UNIT, who have taken charge of the road tunnel, close the wormhole but not before three of the stingrays pass through it. UNIT therefore have to shot down the stingrays, with the aid of some nifty driving by The Doctor in the flying bus.

When all the stingrays are destroyed The Doctor safely lands the bus. On departing Christina asks The Doctor to let her travel with him, but he rejects her because he has no wish to lose another companion. He does though help her to re-enter the bus so as to escape from being arrested by the police for the theft of the gold chalice.

However, one of the passengers, Carmen – who seems to have low-level psychic abilities, has a premonition that visibly unnerves The Doctor…