50 - Name Your Poison, Guy (S4, Ep5)
We watch "The Poison Sky" (Series 4, Episode 5) where the Doctor, Donna and Martha attempt to foil the Sontaran's plan to change Earth into one big baby factory. Whilst the foiling is occurring, Donna does recon up on the Sontaran ship, Martha and her clone prevent UNIT from using nukes and the Doctor convinces a former traitor to blow himself up real good. All this and another blink-and-you'll-miss-it Rose sighting.