Continuing from the end of "The Sontaran Stratagem", the Sontarans launch the attack with their poison gas slowly encircling the Earth. The Doctor—impeded by a Sontaran-controlled clone of Martha—aids UNIT in repelling the Sontarans in the ATMOS factory while Donna infiltrates the Sontarans' warship. He decides to try and burn the poison gas that now encircles the Earth and with the help of th...