Gary and Charlie are on guard duty on Gary's birthday. Charlie starts a rumour about himself that gets out of hand, and Gary's determination to enjoy his birthday is tested by some awful presents and unexpected guests.

Gary is determined to enjoy his birthday, despite the fact that he and Charlie are stuck on guard duty. Gary passes the time by winding up a couple of civilians and singing happy birthday to himself, while Charlie becomes obsessed with guessing the contents of a mysterious letter addressed to Sergeant Thomson.

Gary's attempt to distract him results in a rumour starting about Charlie leaving the army that spreads all around the barracks. Adam and Jacko both drop in to deliver their presents to Gary and to persuade Charlie to stay, but neither gesture gets the intended result. It's up to the final two visitors to the barracks to make Gary's birthday one to remember, and to find out once and for all if Charlie is staying or going.