The Doctor and Donna are on the planet Midnight. Donna just wants to relax, so the Doctor takes the tour to the Sapphire Waterfalls on his own. On the four hour drive he gets to know the people he is travelling with a bit better but suddenly everything changes when their wagon stops. When the Doctor talks to the driver, he can't really find a reason why and then suddenly the knocking starts. There is no known living thing on the planet that can survive the radiation but suddenly something gets into one of the passengers and takes over her body. The being starts to copy everyone and the Doctor knows that it's learning to speak. While the other passengers want to kill the being, the Doctor tries everything to stop them. Soon the others think that the being jumped over into the body of the Doctor and then they want to throw him out of the wagon. The hostess is the only one who sees that the strange being is still inside the woman and not the Doctor and she gives her life to save the others.
This is one of the companion light episodes where we don't see a lot of Donna but more of the Doctor. In my eyes, that's an interesting change from time to time because we can watch the Doctor interact with other people in the absence of his companion. The behaviour of the people reflects how real people would react to something they don't know, something they are afraid of. Not all people would react this way but there are enough who would ad I think this episode shows us a bit about ourselves. The acting is really brilliant, especially when the Doctor and the being speak at exactly the same time. It's also interesting that everything happens in such a small space which leads to more extreme reaction from the people. The ending is quite sad when the hostess saves everyone and nobody has known her name. This episode was something for in between the River Song episodes and the big finale of series 4.