The Poison Sky

Continuing the narration from "The Sontaran Stratagem", the Doctor and UNIT determine that the ATMOS automotive devices, Sontaran in nature, are creating a lethal gas that is slowly filling Earth's atmosphere. The Doctor and Donna Noble are inside in the TARDIS when the Sontaran clone of Martha Jones informs the Sontarans of its position, allowing them to bring it to their orbiting ship. The Sontarans then disable their teleport pods after informing child prodigy Luke Rattigan, their human agent in construction the ATMOS devices, that they never planned on taking him or his students off-planet and stranding him on Earth. Instructing Donna to stay inside the TARDIS, the Doctor learns that the Sontarans, led by General Staal are seeking to transform the Earth into a place hospitable to their species in order to clone more soldiers in their war against the Rutans. The Sontarans further prevent a launch of nuclear weapons by UNIT against their ship through the Martha clone; the Doctor, realizing that the Sontaran ship would be impervious to the weapons' impact, realizes the Sontarans are instead preventing the disruption of their atmospheric conversion.
UNIT attempts to attack the ATMOS factory, but the Sontarans use an energy field to cause the conventional copper-lined bullets in their weapons to expand and jam the firearms, leaving them helpless as the troops are decimated. UNIT re-engage in a counter-attack using non-metallic bullets, and along with the aircraft carrier Valiant blowing the lethal gas away, puts the Sontarans on the defensive. The Doctor cryptically relays instructions to Donna to re-engage the teleport pods while he goads Staal, and uses the distraction of UNIT's offensive to teleport to the factory and discover the real Martha in one of the Sontaran's cloning devices, deactivating her clone in the process. They return to the Sontaran ship to collect Donna and the TARDIS, and then return to Rattigan's estate, finding the teenager distraught over the potential disaster to Earth that he caused. The Doctor collects the necessary equipment to construct his own atmospheric converter, harmlessly igniting the poison gas across the globe and ending the threat. However, the Doctor is aware the Sontarans will not take their defeat easily and teleports with the converted aboard the Sontaran vessel, planning on activating it and destroying the ship. Staal calls the Doctor's bluff, knowing he would also die, but before anyone can react, Rattigan teleports aboard, forces the Doctor back to Earth, and sacrifices himself by triggering the device, destroying the ship and the Sontarans aboard.
As the ATMOS devices are removed from cars and destroyed across the globe, Martha says goodbye to Donna and the Doctor in the TARDIS and prepares to head home. However, before she can leave, the TARDIS suddenly springs to life, locking the doors and piloting itself to an unknown destination as the jar containing the Doctor's severed hand bubbles.