On their wedding day, Charlie and Mary discover that he has forgotten to get his deferment for his National Service and will have to start that day. On his way to the Heather Crest National Service Depot, Charlie meets Horace, another conscript. Horace is a hypochondriac and is just as eager as Charlie to get out of the army. When Charlie arrives at the depot he sees his wife arrive in the back of a truck but is disbelieved by the other men, who think he is hallucinating. Mary has in fact joined the kitchen staff and makes friends with Nora.
Sergeant Grimshaw is about to retire and in his six years at the depot has never had a champion platoon. He bets £50 that he will be able to reverse his luck with his last intake of recruits and win the Star Squad Award. When he meets his new men, he is unimpressed, but Corporal Coffin convinces him to take a mild and sympathetic approach to make sure that he wins the bet.
The new recruits go for a drink and meet Brown, a no-hoper who is undergoing basic training for the third time. While drinking with the others, Charlie finally manages to meet up with Mary. Nora finds herself sympathetic to their plight after falling hopelessly in love with Horace on first sight, although he is initially uninterested in her. She later tries to help Charlie and Mary celebrate their nuptials by finding them an empty room in the depot, but there is a mix-up and Charlie ends up in Sergeant Grimshaw's room instead.
The next day Horace reports sick to Captain Clark, but she finds nothing wrong with him. He eventually rejoins the rest of the men after being kitted out. As they prepare for their first inspection, Charlie realises that two fire hydrants are missing from their barracks. Together with the accident prone Golightly, he heads over to the kitchen and borrow their extinguishers. As Captain Potts and Grimshaw are finishing their inspection, Charlie and Golightly arrive and accidentally set off the hydrants, covering everyone in foam.
Charlie eventually goes to see the Captain and, after some initial misunderstandings, is able to secure one week of compassionate leave so that he and Mary can finally spend their wedding night together. The men continue their basic training and find themselves frequently at odds with the university-educated Bailey, a fellow recruit who considers himself superior to the other men.
After his return, Charlie soon gets into trouble when, during a training session on how to assemble a machine gun, he and Horace are distracted by Mary and Nora. Mary and Charlie decide to help Nora, and tell Horace that he has to report to the kitchen for fatigues following another accident while training. Bailey eventually starts to participate with the rest of the group when Brown asks for help in completing the course. After visiting the doctor on an almost daily basis, she takes Horace to see a team of specialists, who eventually convince him that he is in perfect physical health. While undergoing a psychiatric evaluation, Horace realises how drawn he really is to Nora. Brimming with newfound confidence about his health, he heads back to the kitchen embraces Nora.
Captain Potts plots the downward progress of the platoon on a graph and on the last day of training tells them they are the worst he has even seen. Brown overhears the Sergeant and the Corporal bemoaning his sad fate with this last squad. The men realise how generous the Sergeant has actually been with them and decide to make a final effort. On the parade ground they all finally come together as a unit and win the Star Squad Award. On Sergeant Grimshaw's last day, the men thank him and give him a Ronson cigarette lighter as a going away present.
