With Earth's atmosphere becoming toxic, UNIT determines that the source of the signal is coming from space.  Donna hides in the TARDIS but when the Martha clone finds out about it, she and it are transmatted onto the Sontaran ship.  UNIT plans to launch a nuclear strike against the Sontarans but Martha, having hacked into the NATO computers, keeps overriding the launch.  With the air getting more poisoned by the minute, can the Doctor live up to his trust in Donna and her grandfather and save the day?