4.5 - The Poison Sky

From IMDB:
The Sontarans launch the attack with their poison gas slowly encircling the Earth. They've also taken the TARDIS to its headquarters but don't realize that Donna is inside. The Doctor realizes that the Sontarans have cloned Martha. He knows that fighting the Sontarans is of little use as they never surrender and are prepared to fight until they win. He decides to try and burn the poison gas that now encircles the Earth and with the help of the Sontarans teleportation system, send them a little surprise as well. At the end of it all, both Martha and Donna have to decide if they want to stay with the Doctor.

97 out of 109 (89.0%) raters say this story requires a previous story.