In 2009, the Sontaran Empire was losing its 50,000 year war with the Rutans. The Tenth Sontaran Fleet, under Staal's command, attempted to find a suitable planet for terraforming into a new breeding world in order to replenish the Sontaran forces, and chose Earth.
Allying with genius teenager Luke Rattigan, the Sontarans developed the ATMOS: a device capable of completely eliminating CO2 emissions in Earth vehicles. The ATMOS was so successful that half the cars on Earth were fitted with the devices, but the public were unaware that ATMOS also emitted toxic gases that the Sontarans needed in order to convert the Earth's atmosphere for their purposes.
With his plan jeopardized by the Doctor, Staal advanced his stratagem and activated all of the ATMOS devices worldwide, saturating the atmosphere. However, the Doctor reconfigured an atmospheric converter that he took from Rattigan Academy and used it to ignite the gases emitted by ATMOS. The sky was set ablaze and the toxins were all burned off in a matter of seconds. In response, Staal commanded his troops to take Earth by force.
The Doctor used Rattigan's teleporter to beam onto Staal's ship with the atmosphere converter, which he had reconfigured to ignite the air onboard the Sontaran warship. He gave the Sontarans the chance to leave, but Staal mocked him and refused. As the Doctor prepared to throw the switch on the converter, Rattigan adjusted the teleport link and beamed the Doctor back but beamed himself up in his place. Rattigan then threw the switch and ignited the air, destroying the Sontaran ship.