Martha phones the Doctor and calls him back to Earth. Something strange has happened and 52 people have died at the same time all over the planet. The thing they all had in common was the Atmos device in their cars. When Martha and Donna meet for the first time, the Doctor thinks that they won't get along very well but they seem to be the best mates from the start. The Doctor investigates the Atmos device but can't find anything unusual at first but he has this feeling that there has to be something more. So while he goes to the Rattigan Academy Martha stays with UNIT at the Atmos factory and tries to find out more about the workers who seem to work for 24 h a day. Donna visits her family and can share her adventures with the Doctor with her grandfather. The Doctor finds a teleport device when he visits the Rattigan Academy and now he knows that the Sontarans are behind the strange happenings but what do they want to achieve. With a clone of Martha and all of the Atmos devices activated, the plan of the Sontarans seems to work.
I think it's great to have Martha back who's now working as a doctor for UNIT. The fact that you don't know who's behind of the Atmos device and everything else is very clever and gives you some time to ponder which alien it could be. The Sontarans have appeared before in Doctor Who but I think this is the first time they are back in the episodes since 2005. They really look a bit like potatoes when they take their helmets off but they are interesting although they live for the war. In this first part of the two parter you still don't really know what their plan is, only that they've activated all the Atmos devices and a gas comes out of the cars. They call it a weapon but nobody knows what it'll do on Earth. Even the Doctor is a bit clueless and doesn't know what to do to help the people but we know he'll find a way.