There is to be a parade through Walmington-on-Sea to commemorate Spitfire Week and, after watching a newsreel, the platoon decide it would be a splendid idea to have a mascot. Private Sponge farms sheep and the men try to catch his ram but it eludes them every time. Walker provides a goat but it is a skinny, pitiful creature which eats Mainwaring's five pound note. So there is no mascot for the parade, which turns into a race as the platoon, Hodges and his wardens, the verger and his Sea Scouts and the volunteer nurses all march faster and faster to be at the head of the procession until they all break into a run.

The platoon is to take part in an exercise to supposedly place a bomb in a windmill occupied by the Eastgate men. It is decided that, as the mill is by water, Frazer, in an ancient diving suit left him by a friend who died of the Dreaded Bends, will push Jones, disguised as a log, up to the mill, where Jones will discharge the 'bomb' and, thanks to clever decoy plans used to fool Square, Jones successfully makes it, only to get caught up on the windmill's sails.

Dissent in the ranks follows after a foot inspection as a result of which Mainwaring decides the platoon need their feet toughening up and proposes bare foot football as well as a twenty mile route march. To prevent the march the men hatch a plan whereby they will go into the shoe shop from where Mainwaring has ordered his new boots and substitute a size smaller which will pinch his feet and put him off the idea of a long march. What they had failed to appreciate is that he already had a pair in his own size in for repair, which the shoe-maker returns to him before the march.

The platoon are on night watch in a hut by the beach waiting for Pike to bring their tea but Pike has got himself tangled up on barbed wire on the beach in the middle of a minefield. Whilst waiting for an engineer to come and free him, Mainwaring and Frazer mount a rescue attempt of their own, to be beaten to it by Godfrey. Pike is freed and when the engineer turns up he tells everyone that Pike was never in danger because the minefield was some two hundred yards further along the beach - though Jones knows better!

The platoon is to guard a prisoner of war camp housing Italian soldiers and Mainwaring is suspicious because Walker seems rather too friendly with them. However Walker explains that he uses them as cheap labour to mend radios for his customers and they go backwards and forwards in and out of the camp via a tunnel which they have dug beneath the stove. When an inspection is called for and only half the prisoners are available Jones, who is doing the head count, makes them go round twice but the plan is rumbled when the missing half turns up.

On return from London Mainwaring is shocked to find that almost all of the platoon are playing in a darts tournament against Hodges and the air raid wardens, which they refuse to leave, despite losing. Wilson is not happy because Hodges has bought Mavis a drink and seems to be getting overly familiar with her. When a call comes through that a suspected I.R.A. terrorist is in the area, however, the men pull together - eventually - and Wilson proves to Mavis and the others that he is the hero of the hour.

Jones, Fraser, Godfrey and Pike are trapped in the lighthouse tower at night when they accidentally illuminate the entire town for enemy bombers.

An elderly ex-soldier called Clarke comes to join the platoon and, by coincidence, it turns out that he and Jones were in the same regiment in Sudan back in the 1890s. However he paints a picture of a cowardly Jones who left him to his fate when he was captured by Dervishes, and soon after Jones receives two feathers, signifying cowardice, through the post. The corporal then tells his story in a flashback where all the participants look like members of the present platoon. He and Clarke were indeed captured but he rescued Clarke and kept quiet about his affair with the colonel's wife. Mainwaring is satisfied with Jones's account but before he can confront Clarke, Clarke leaves town, returning his uniform.

After Mainwaring announces that women are to join the Home Guard several lady friends of the platoon come forward, as does Fiona Gray, an elegant, quietly-spoken widow who has brought her mother down from London to escape the bombs. Mainwaring is very taken by her and contrives to have morning coffee in a local tea-shop she regularly uses, where they are interrupted by various platoon members. They are seen out a lot together and Wilson tries in vain to tell Mainwaring that he is the subject of gossip. Fiona realizes this and so decides to return to London without telling Mainwaring, who arrives at the station in time to say goodbye to her.

The ARP wardens challenge the Home Guard to a cricket match.

Wilson is promoted to Manager of the bank in Eastgate, where he joins their Home Guard at the rank of First Liuetenant. His new life in Eastgate is cut short, however, as the bank is bombed the day he arrives.

Once more, the civic vicar's hospitality for both Home guard and ARP forces the rival corps' men to cohabit in the church. This time captain Mainwaring and chief warden Hodges even have to share the vicar's desk at the same time. Their men try everything to push the others away, and end up lighting the chimney so ineptly that the church tower catches fire, making it a prime air raid target. As that would render them the laughing stock of the county, they decide to put out the fire on their own, with their usual mixture of bravery and utter bumbling incompetence.

The platoon gets its turn at weekend training, for a bomb course. Captain Square convinces Mainwaring that officers shouldn't sleep with the OR (other ranks) so he puts up blankets for him and Sergeant Wilson, just after he forbade the others to voluntarily 'spoil him'. Next Square insists a captain can't join his men in the canteen for beer, rationed to two pints for safety, which the men, especially Frazer, take for disloyalty. In the Officer's Mess Mainwaring naively volunteers to be 'made a cardinal', a rather cruel drinking game...

Already peeved because Captain Square has drawn attention to the fact that he is the only member of the platoon not to have medals - except Pike, who has his scout's badges - Mainwaring is happy to take on the challenge of beating Square's Eastgate platoon in an initiative test, a race to an old tower where the winner is the first to fly their flag. Jones gets a touch of malaria whilst driving the van and Hodges deliberately stops them to give Eastgate the advantage, after which a stand-off on a narrow bridge leads to Hodges getting pushed into the river. The Walmington men are the first to the tower though Mainwaring accidentally pulls the rope off the flagpole and Jones, drunk on the tonic wine Godfrey gave him to cure his malaria, almost falls to his death. The Eastgate platoon are the first to unfurl a flag but it is the Walmington platoon's flag, giving Mainwaring's platoon victory. Afterwards Mainwaring learns from Walker how this came about.