It’s Christmas Eve and the team are joined by Corporal Gordon House (Towerblock) as he begins his tour of duty. Nick has to try and get him up to speed, but as Towerblock finished top of his class on the High Threat Course, he believes that he knows everything, but he soon discovers that things are very different in the field - even on Christmas.
Towerblock is showing off his smart new Glock 17 pistol. Nick believes that he needs to try and prove to him that he can pass a weapons-handling test and get one himself, but things don't go according to plan. Simon attempts to gain the respect of others by borrowing the Lt Col's copy of Sun Tzu's Art Of War, landing Bird in trouble after she becomes drawn into his obsession with the book.
After a blast from Nick’s past shows up, it throws him off his game which seems to also have a dangerous effect on the rest of the team.
Nick wants some downtime so that he can focus on making a forensic profile of the Taliban bomber, but his team are constantly bugging him. He is over the moon to discover a disused roof terrace. but soon sombody else is also wanting to use the space. Towerblock comes up with a plan to scare Bird with Rocket's new pet rat.
After Mac and Rocket set fire to the Det, Nick decides to take the blame and is forced to look after a Government Minister who is visiting the base as his punishment. He soon regrets asking Mary to look after the visitor when she unexpectedly takes a liking to him.
Bird takes action when she's fed up of hearing about Mary from Nick. Simon tries proving himself on a computer game. Towerblock pimps the Lt Colonel's car out.
Graham the robot is stolen from under the noses of the squadron. Bird finds herself in an uncomfortable conversation with Mary about Nick. Simon's lack of trust comes back to bite him when he loses a part of his rifle.