Dad's Army: A cultural smash'n'grab that goes through the motions

You don’t.

If you’re director Oliver Parker and screenwriter Hamish McColl, you pepper a flimsy plot that would barely stretch to one TV episode let alone 100 minutes with the show’s catchphrases and pray our abiding affection for the characters will compensate for long passages without a discernible punchline.

Original cast members Ian Lavender and Frank Williams are conscripted to cameo roles to heighten the whiff of nostalgia.

Limp innuendo-laden banter about sausages barely merits a smirk, pratfalls are predictable and a terrific ensemble cast of gifted comic actors go on patrol without an arsenal of decent one-liners.

From uninspired beginning to muddled end, it’s a cultural smash’n’grab that goes through the motions and will ultimately be remembered as a badly missed opportunity.

England, 1944.

The Second World War is on a knife edge and in the cosy community of Walmington-on-Sea, blustering bank manager George Mainwaring (Toby Jones) proudly leads the local Home Guard.

His hapless rank and file includes Sergeant Wilson (Bill Nighy), Lance Corporal Jones (Tom Courtenay) and Privates Frazer (Bill Paterson), Pike (Blake Harrison), Walker (Daniel Mays) and Godfrey (Michael Gambon), a mild-mannered soul who frequently drifts off into his own world.

The fate of the Home Guard hangs in the balance when Colonel Theakes (Mark Gatiss) reveals that he intends to sort the military wheat from the chaff and “Walmington feels chaffy.”

Soon after, Mainwaring learns that a German spy has infiltrated the town and is transmitting secrets back to Berlin.

This search for a traitor coincides with the arrival of glamorous magazine writer Rose Winters (Catherine Zeta-Jones), who intends to pen a flattering article about the heroics of the Home Guard.

George is smitten and finds Rose most charming and agreeable.

“They said that about the Ripper,” coldly retorts Mrs Mainwaring (Felicity Montagu), hard-nosed leader of Walmington-on-Sea’s women’s auxiliary army, which includes Pike’s mother (Sarah Lancashire) and Walker’s sweetheart Daphne (Emily Atack).

Dad’s Army opens with a limp set piece involving a stand-off between the Home Guard and runaway livestock.

“We’re supposed to be locking horns with the Hun not Bertie the bull!” despairs one of the men, echoing our mounting frustration.

Jones lightens the darkening mood with a few moments of physical humour, including choking on a slice of cake, while Nighy relies on his usual snorts and tics for merriment.

Montagu, Lancashire and co bring a diluted degree of girl power to proceedings that might be dismissed as tokenism without their characters’ pivotal involvement in the hare-brained and lacklustre denouement.