The Sontarans try to choke the Earth and the Doctor doesn't know exactly why. When he returns to the mobile UNIT headquarters Donna stays in the Tardis. The false Martha enables the Sontarans to get hold of the Tardis, so the Doctor can't escape from Earth. But they don't know that Donna is in the Tardis and now on board of the Sontaran ship and Therefore she is the only one who can help the Doctor. When she calls him, he tells her what she has to do to activate the teleport again. He knows she is in danger but it's the only way he can save her and the Earth. Meanwhile the Doctor has found out about the false Martha and he saves the real one. Together again, he, Martha and Donna are at Rattigan academy where they find something to clear the atmosphere of the Earth but will it work? And how can they stop the Sontarans?
I think this was a great second part in this two-parter. The Doctor probably suspected from the beginning that Martha wasn't Martha anymore because you could see that he tested her a couple of times. I guess he has spent enough time with her in the Tardis to notice the differences even though the Martha clone had her memories and everything. The episode was quite dramatic with Earth choking slowly in the fumes of the cars but it also had it's funny moments. I just loved it when Donna was on the Sontaran ship and stood before the door which you could only open with three fingers and the Doctor hat to tell that she had three fingers. Just brilliant. I enjoy seeing Wilfred in Doctor Who. He just adds something unique to the episodes he's in. I think the episode had a good ending but it was somehow clear that the Doctor would survive.