This is what what happens when the young lampoon a subject revered by the old. We are talking about a British Intelligence operation that deceived Hitler into moving 90,000 troops out of Sicily and so saving thousands of Allied lives. Surely this is a subject worthy of sober drama?

However, the five-strong company SplitLip (three of whom double as writer performers, and one of whom is glam-rock composer Felix Hagan) have instead created a madcap musical. And you can’t blame them really. The operation that gives this show its name used the body of a dead tramp, gave him the fake identity of a Royal Marines officer and dumped him off the coast of Spain with a briefcase of pretend secrets in his possession. The documents showed that the allies would invade German occupied Sardinia not Sicily.

From this far-fetched fact a movie was made called The Man Who Never Was. And now there is this show — first seen in 2019 at the New Diorama Theatre and returning here in expanded form. Too expanded, in fact. The musical could do with at least 20 minutes shaved off the running time not least because the pitch and pace is so frenetic.

Much of the comedy targets the group of upper class intelligence officers whose job was to think up deceptions that would shorten the war. One of them was Ian Flemming. This results in gags that are mainly of the broad brushstroke variety epitomised by a song that pokes fun at the nation’s public school-educated who presume they are born to rule.

In Donnacadh O’Briain’s breathless production the familiar tropes of class-conscious comedy quickly frays. But although initially thinly drawn, these characters eventually deepen. And the sense that this team of buffoons care deeply about saving lives allows the show’s tone to switch from shallow irreverence to something like respect for the plan’s objective.

Granted, the score often feels like the spawn of Hamilton in the way it deploys hip hop in a period well before it existed. But just as often there is original songwriting on offer. And for one of these the pace slows to allow for a genuinely moving reality check.

The song is called Dear Bill and it arrives in the scene when the unit are attempting to cobble together a convincing letter from the fake officer’s wife to be inserted among his effects. Hester (Jak Malone), the waspish head of the unit’s secretariat, takes over the floundering attempt with the observation that what soldiers want to hear about when they are away is normal life back home.

This apparent spinster then gives one such an example apparently drawing on her own experience from the previous war. She describes such subjects as the roses that fail to grow and the children who misbehave. Each banal detail is worthy of a sob.

Jak Malone carries the song like the fragile gem that it is and a stillness descends on the auditorium. Then we’re off again in madcap mode led by Natasha Hodgson as the unit’s arrogant head Ewen Mantagu —in real life Jewish, but that’s not mentioned here — and David Cumming as the diffident boffin Charles Cholmondeley who thought up the dastardly plan.

Both are terrific, particularly Cumming who sweats the genius’s painfully gauche personality through every pore. And somehow the ensemble give this odd piece of history the batty show that it deserves.