Field Major Kaarsh was a Sontaran. In 1605, a Rutan ship which had gone missing in the 13th century sent a distress beacon from London, near Parliament. Kaarsh and his men started searching for the ship, which contained doomsday weapons the Sontaran Empire could use against the Rutan Host. While searching for this weapon, Kaarsh placed Black Rod under mind control and arrested Rory Williams as a prisoner of war, mistaking the latter as an agent of the Rutans. Both later escaped.
When the Eleventh Doctor took Parliament into orbit, Field Major Kaarsh took Charlie for questioning about the weapon, not realising the one he had dropped was one. Rory and his wife Amy escaped with Charlie, but was cornered by Kaarsh and Lady Winters. The Doctor handed each of them two doomsday weapons, one targeted at Sontarans, and the other against the Rutans, with the risk that the Sontarans could target themselves. Kaarsh laughed at his nullification of the weapons, stating that the Doctor made the war "honourable". (GAME: The Gunpowder Plot)