The XK-5 space freighter was a spacecraft used by humanity in the 20th century. For its first test, Guy Crayford travelled into space where he went missing and was presumed to have hit an asteroid. In actuality, he had been captured by the Kraals who wanted to invade Earth. Their Chief Scientist, Styggron, convinced him to assist them in their cause.
The Kraals planned to use the freighter to travel to Earth with the Kraal androids on board, land, have the androids release the Kraal virus and then Marshal Chedaki would enter the fray with the invasion fleet. The Fourth Doctor and Sarah Jane Smith journeyed to Earth aboard it, passing through a space-time warp. They travelled down to the surface in retro tubes carried aboard the XK-5.
Devesham Control allowed the freighter to land, having received a communication from Crayford saying he would be returning. It came down by the Space Defence Station. Styggron stayed inside, hoping to be safe from the contagion. Sarah entered and freed Harry Sullivan and Colonal Faraday, who had been replaced in the station by androids. The Doctor reprogrammed his android double to attack Styggron. It knocked him onto a contained with the virus inside and he was killed. (TV: The Android Invasion)