D I McMillan oversees the police as they arrive, but upon glancing at the bus as it pulls away, recognises the woman he is looking for and quickly clambers into a car to give chase.
Onboard the bus the Doctor enthuses about Easter before breaking off to examine a botched scanner he is carrying in his pocket – whatever he is looking for is fast approaching.
The bus enters a tunnel and McMillan orders the traffic authorities to seal it off. Sure enough police cars arrive at the other end to stop the bus in its tracks.
However, the bus does not make it through the tunnel – the Rhodium particles the Doctor is scanning for amass and he realises something is about to go wrong. As an old woman at the back of the bus complains to her husband that she can hear voices, the Doctor warns the other passengers to hold tight. Suddenly everything outside goes dark and the vehicle begins to shake violently. Something strikes the roof and the windows shatter. Back in the subway, McMillan reports to the officers at the other end that the bus has disappeared.
It arrives in a desert, a seemingly endless landscape of sand dunes. The Doctor and a few of the other passengers step outside, unaware something is watching them via a scanner screen. The elderly woman again confesses that she hears voices – now they are all around her. Her husband asks her whom they belong to and she cryptically responds by claiming they are the voices of the dead.
Outside the Doctor examines the sand whilst the passengers marvel at the three suns suspended in the sky. The driver inspects the damaged upper deck and reveals that the vehicle has become stuck in the sand – they are stranded. The museum thief steps out onto the sand and watches as the Doctor tastes it, sensing something odd in the composition.
Recalling the device the Doctor was holding when the bus left Earth, another of the passengers, a young man, accuses him of causing what has happened. He responds by claiming that he was simply tracking a wormhole, which for some inexplicable reason swelled to a giant size and sucked the bus through. He demonstrates by throwing a pebble at the space behind the bus. It hits an invisible barrier, which flares with colour – both there and at the other side in London. The driver assumes that they can simply walk through the portal and home again. Despite the Doctor’s warnings he does so and is immediately reduced to skeletal form, disappearing into the wormhole and emerging in a heap back in London. Witnessing this, McMillan realises he is out of his depth and calls in UNIT.
Back on the sand planes the Doctor explains that the bus protected the passengers from the wormhole as it is made of metal – it took the brunt of the damage. Now they must find a way of digging it out of the sand and driving it back through so that they can all get home. At that point the female burglar decides to take charge of the group and ushers them all back inside the damaged bus.
Once there they all identify themselves. She is Christina and the young man who questioned the Doctor earlier is Barclay. There is also another young lad, named Nathan, who is comforting a distressed lady named Angela. The elderly couple at the back are Carmen and Lou. The Doctor then begins discussing the wormhole. He initially claims it was an accident that they drove through it, but Carmen insists someone placed it there; the voices seem to be telling her so. The Doctor identifies that Carmen has a low-level psychic ability, exacerbated by the alien suns outside.
She talks of the voices she can hear and begins to identify them. She claims they are death, and that they are getting closer. The other passengers begin to panic but the Doctor regains calm when he asks them each to focus on what is waiting for them back home, which is where he promises to return them to.
In London, UNIT troops arrive at the tunnel. They sweep the area with precision, surrounding the opening of the wormhole, guns trained. Captain Magambo takes charge, clearing the police (including D.I. McMillan) from the scene. Standing before the tunnel, Magambo orders her men to shoot anything that appears hostile on-sight.
On the other side of the wormhole, Nathan and Barclay are at work digging the bus free from the sand, with a little help from the tools Christina keeps in her rucksack. Angela attempts to start the bus but the engine is damaged, clogged with sand. Barclay, who has some knowledge of mechanics, begins work fixing it whilst the Doctor, followed closely by Christina, walks off to one of the sand dunes – still being watched by some alien creature on its scanner.
The Doctor questions why Christina, who seems equipped for any possibility, would be so scared by the police alarms they heard earlier, but she does not offer him any answers. Instead she asks him about Carmen’s prediction, and he admits to sensing something unsettling in the air. Climbing the dunes Christina offers him her full title – Lady Christine de Souza – and the pair are introduced properly. However, this is quickly ended when they spot something on the horizon – a fast-approaching storm. Christina assumes it must be a sandstorm, but the Doctor is not so sure.
Inside the bus, Carmen begins to recount the voices again as the Doctor and Christina return. The Time Lord grabs a nearby mobile phone and, with the help of the Sonic Screwdriver, phones UNIT HQ. He is put through to Captain Magambo, telling her that he needs to examine the wormhole but has no equipment with which to do so.
She refers him to their on-site Scientific Adviser Dr Malcolm Taylor; a professorial-looking man sat in a mobile laboratory. Magambo passes on the call to him, who is hugely excited by the prospect of talking to his hero. The two men discuss the wormhole and it transpires that Malcolm is already attempting to measure it. The Doctor gives him further instructions and cancels the call, returning to the dunes to take a photo of the storm clouds with the phone. He spots a flickering in amongst the clouds, as if something metal were there as well. In the bus Carmen reveals more about the clods; she implies there is something within them, something that will devour everything in its path.
Back on the dunes Christina and the Doctor are confronted by the thing that has been watching them. It is a humanoid figure in stance, but with the head of a fly. Pointing a gun at them, it leads them away to its ship, which has crashed some distance away – like them it is stranded. Onboard they find another of the creatures, which are called Tritovores, and are accused of causing their ship to crash. The Doctor tells them that he and his friends are also trapped, and gains the trust of the aliens. He turns to their computer systems and, after fixing some of the broken circuits, manages t send out a probe to investigate the approaching storm.
He then accesses the navigation computer and determines that they are on the planet San Helios. The Tritovores explain that they came to trade with the planet, but could not find the main city. They discover that the desert is the city, reduced to sand. The voices in Carmen’s head are the voices of the billions of people who once populated the planet, all dying in the dust.
A call comes through from Malcolm, who tells the Doctor that the wormhole is getting bigger; it is now four miles in circumference. Magambo has grounded all flights across the city, but is worried the portal may now pose a threat to Earth security. Another call interrupts their conversation – it is Nathan. He explains that the bus’ wheels have been dug free but now the engine, which they kept turning over to see if it was fixed, is out of petrol.
Again the conversation is interrupted –the Tritovore probe has reached the distant storm. Just as the Doctor feared, they clouds are not dust; they are a swarm. Millions of semi-metallic stingray-like creatures are soaring through the air, approaching the bus, the crashed ship, and the wormhole.
Studying the analysis of the creatures made by the probe, the Doctor deduces that the creatures create a wormhole in time and space, fly through it, devour everything they can find and then move on to the next one. The size of the swarm and the speed at which they travel generates enough power to open the portal. Now they have gained enough momentum circling San Helios, they are opening the next wormhole – which leads to Earth. The metal he and Christina saw in the clouds was the creatures’ exoskeletons, which protect them when they pass through the wormhole.
Whilst Carmen reveals another prediction – that ‘the girl will fly’ – Christina asks why the Tritovores crashed since they did not pass through the wormhole, and they reveal that their drive system stalled ten miles up in the air. They take the Doctor and Christina to the cause of the problem – the gravity well, where the Doctor realises that the crystal nucleus of the well could be used to power the bus. However, the crystal is at the bottom of the well. He runs off to the service shafts, handing Christina a communication earpiece so that they can stay in contact whilst she remains at the foot of the well.
However, Christina has no intention of staying put. Whilst the Doctor works at the compute system, she harnesses herself in and sets up the wire she used to rob the museum. The Doctor soon realises what she intends to do and hurries back to stop her. She is halfway down the well when he jams the pulley system. He reveals that she is about to hit the electrified security system, and instructs her on disabling it before reluctantly allowing her to descend once more, albeit at the slower pace.
Finally given the chance to talk, the Doctor reveals that he is a space-time traveller, and recounts some of the many places he has been. As he does so he opens Christina’s backpack to reveal the item she took from the museum – the Cup of Athelstan. At last Christina reveals her motives – her father has been made bankrupt and she wishes to regain her fortunes. The Cup is worth enough to supply her with that and more.
She eventually reaches the bottom of the well and finds the crystal. However, something else is waiting down there – one of the swarm. The Doctor realises that it must have flown into the engines of the Tritovore’s ship and caused the crash. Christina quickly grabs the crystal and it’s metallic holder and ascends back to the main deck, but the creature is quickly on her tail. She manages to stop it by reactivating the security blockade on her way up, and returns to the Doctor safely.
The two friends try to convince the Tritovores to return to Earth with them, but when another creature from the swarm breaks into the chamber the aliens make the mistake of trying to fend it off and are devoured. The Doctor and Christine flee back to the desert and head back to the bus, as the swarm gets closer. On the way Malcolm calls again with news but the Doctor has no time to talk. He reaches the bus and explains to Christina that the crystal she retrieved is useless – he needs the clamps that were holding it in place. He throws the gemstone away and begins fixing the clamps to the wheels of the bus.
He and Christina board the vehicle and the Doctor takes to the driver’s seat, fixing the last clamp to the steering wheel. He phones UNIT again and asks Malcolm to find a way of closing the wormhole as soon as the bus returns. Once again Malcolm is one step ahead with an idea, but Captain Magambo is more concerned with what might be following the bus through the ever-increasing portal. The Doctor hangs up to avoid explaining but Magambo is not deterred. She orders her troops to cover the wormhole with their weapons aimed and ready.
On the bus, the Doctor’s dismay, the clamp on the steering wheel is rendered incompatible, unless he can meld the two systems together with a non-corrosive, malleable metal – something gold. Begrudgingly Christina hands over the Cup of Athelstan, which he proceeds to beat mercilessly with a hammer so that it will fit into the clamp.
Back on Earth Malcolm readies his solution for closing the wormhole and prepares for the bus’ return. However, Captain Magambo orders him to close the portal immediately to prevent any threat making its way through. He refuses, not wanting to leave his hero stranded on an alien world. Unperturbed, Magambo points her gun at him and repeats her orders.
With the system working, the Doctor starts the bus. The gravity clamps lift the vehicle up into the air, and slowly but surely it turns around. As the swarm finally reaches them the passengers are propelled forward into the wormhole, through the darkness of the void beyond and back to Earth. However, they are soon followed by three of the swarm creatures, which Captain Magambo – having heard of the bus’ return and gone to investigate – orders her men to open fire at.
The Doctor phones Malcolm and tells him to close the wormhole. He immediately hangs up and starts the process, but something short-circuits. Eventually the Doctor gets through the Malcolm again and gives him the key to fixing the system. He hurriedly computes it in and the wormhole closes, leaving the rest of the storm back on San Helios.
Outside, two of the creatures are blasted out of the sky, but one remains soaring near the airborne bus. With some nifty manoeuvring the Doctor bats it out of the way and UNIT are able to destroy it. Crisis averted the Doctor returns the jubilant passengers back to the ground, welcomed by applause from the UNIT troops. The passengers, Christina included, are led away for debriefing and the Doctor approaches Captain Magambo. Malcolm is there to welcome him too, but is quickly returned to his station, leaving them alone. The Doctor explains that the swarm will still be able to create another wormhole, but he shall endeavour to shift them onto uninhabited planets to avoid any more trouble. Magambo then reveals that the TARDIS has been found and brought to the scene so that he can leave promptly. He thanks her and they bid each other a fond farewell.
Christina manages to break away from the rest of the passengers and approaches the Doctor, asking him to take her with him when he goes. He refuses, telling her that he has travelled with many people before, and he has lost them all; he will not go through that pain again. He watches as D.I McMillan arrives and arrests Christina, before leading her away to a nearby police car.
Carmen and Lou then approach the Doctor and wish him goodbye, but Carmen carries a far more sinister message for him. Telling him to look after himself she warns him:
“You be careful, because your song is ending, sir…It is returning. It is returning through the dark. And then Doctor…oh, but then…he will knock four times.”
And with that she leaves. The Doctor, clearly struck by her words, looks coldly into the distance before turning to watch Christina being put in the back of the police car. He quickly pulls out his Sonic Screwdriver and zaps her handcuffs, releasing her hands. She hurriedly escapes the car and runs off to the bus, D.I. McMillan close behind.
Unable to do anything, he watches as the bus ascends into the sky with Christina at the wheel. She navigates over to where the Doctor is standing and tells him that they would have made a great team. However, the Doctor knows that already were back on San Helios. He watches as she flies off into the night, and then turns and steps back inside his TARDIS.