SHADO member Paul Roper is being blackmailed by a traitorous SHADO medic to provide information to the Aliens. He initially refuses, but when he fears for his wife's safety, he complies. Straker puts him under surveilance after his stress levels are revealed to be alarmingly high during a routine SHADO medical check. Roper is caught giving information out over the phone, which is later discovered to be a flight path which will allow a UFO to approach Moonbase undetected due to interference from the Sun. In order to "even the score", Roper volunteers to defend Moonbase himself. But what are his chances, alone on the lunar surface and armed only with a handheld rocket launcher?