Lady Christina steals a valuable object from a museum but before the police can catch her she jumps onto a bus. The Doctor gets onto the same bus because he is following some strange readings. The police has sealed off a tunnel to stop the bus but the bus never reaches the end of the tunnel and ends up on an alien desert planet instead. There, the bus is stuck in the sand but they need it to return to Earth through the wormhole. On Earth, the police calls UNIT for help and soon they have sealed off the area and are ready to face whatever comes through the wormhole. When the Doctor notices a huge sandstorm coming closer he calls UNIT on Earth and together they try to bring the bus back. The Doctor and Lady Christina explore the planet and are captured by the Tritovores. Soon, they find out that these beings aren't dangerous and that they can help them to go back home. It becomes obvious that it's not a normal storm that's coming their way but an alien life form that can destroy whole planets. The Doctor can't save the Tritovores but with some of their technology he is able to bring the bus back to London and UNIT can close the wormhole. After saving the people, the Doctor leaves alone in his Tardis.
The Doctor really has fun in this episode and he has found an equal in Lady Christina. It's just fun to watch their banter but he is just not ready for another companion in his life after he has lost Donna and Rose. I'm also not sure if I would have liked Lady Christina to be a companion for more than one episode. For me, she is more a self-centred person than somebody who helps others. I don't know if it would have worked between her and the Doctor for a longer time. The swarm is a quite scary monster because they just eat everything that is in their way. They aren't evil but I think their natural habits make them scarier than some other monsters. The Tritovores are not the most beautiful aliens we have seen so far but they are quite interesting nonetheless. I like it that the Doctor has kind of more companions than the normal one in this special and they all work really good together. The moments when the Doctor finally meets the people from UNIT are great as well and it's nice that he helps Christina to escape in the end. One thing that's amazing is that they've filmed most of the stuff on the alien planet in a real desert which gives the episode a bigger scale.