Aided by the Rattigan Academy, the Sontarans establish a covert base at the mysterious ATMOS Factory. Can the Doctor stop the Sontarans from unleashing a potent terror that will bring the world to its knees?

The Sontarans continue their evil plot and begin to "choke" the Earth. The Doctor and Donna, now teamed with Martha have to stop them! Can they save Earth before it's too late?

Whilst attending a carnival on the Chino-planet of Shan Shen, Donna is cajoled into having her fortune read, where her past is carefully examined. With the Doctor missing, Donna must work with Rose, a traveller from a parallel universe, to prevent darkness encompassing the whole of the universe.