A species of green aliens used a ship that they brought to Earth in order to search for Douglas Henderson, who unknowingly was a living weapon. They planned to use the weapon to end a war with a neighbouring planet, which was home to a similar species.
Their plans failed when Douglas activated himself, disabling both the green alien's and yellow alien's ships. The Tenth Doctor left to tow both fleets back to their homeplanets. (COMIC: The Big, Blue Box)