Emotionally reserved and exhaustively footnoted World War II strategizing is the sort of thing Colin Firth, Matthew Macfadyen, Jason Isaacs, Penelope Wilton, Kelly MacDonald and Mark Gatiss could pull off in their sleep. That’s mostly what they do across the overly expansive Operation Mincemeat, which premieres Friday in theaters and Wednesday, May 11 on Netflix.

The film is based on a real-life longshot of British intelligence, which used a corpse near Cádiz to cause a chain reaction that, if successful, would cause Hitler to divert troops to Greece and set up a successful Allied invasion at Sicily. 

Firth and Macfadyen are, respectively, Ewen Montagu and Charles Cholmondeley, who forge an occasionally antagonistic partnership as the plan’s master architects. Isaacs is John Godfrey, a Royal Navy officer who believes Mincemeat is a fool’s errand and targets the distasteful political affiliations of Montagu’s brother, Ivor (Gatiss), in an effort to sabotage it. Wilton and MacDonald are the scheme’s distaff designates, the former as Hester, Ewen’s longtime aide de camp, and the latter as Jean, an MI5 clerk whose photograph plays a pivotal role in the plot and whose romantic longing for a very married Ewen complicates its success. 

Any successful deception is seasoned with truth, and the drama of Operation Mincemeat mostly avoids the mission’s ticking clock for how its plotters projected their own personalities and peccadilloes about valor, romance, regret and despair onto the dead man’s persona. The screenplay hails from Michelle Ashford, who worked on HBO’s The Pacific and Showtime’s Masters of Sex, of which Mincemeat feels like a meld where the volume and va-voom has been reduced for the Masterpiece crowd. Meanwhile, John Madden (Shakespeare in Love) directs with a perfunctory professionalism in which the precise re-creations of British-basement war rooms is the only memorable detail.

The problem is that Mincemeat rarely diverges from the bone-deep dreariness its conspirators feel amid the daily drudgery of war, engorging itself past the two-hour mark with bits of explanation and exposition that eventually feel endless. Subplots fizzle as quickly as they’re introduced, a section devoted to an official burial for the patsy corpse dawdles, the final act basically asks all these British ringers to hover nervously over teletypes, and there’s never anything too juicy in will-they-or-won’t-they bits with Ewen and Jean so as to avoid smearing reputations or setting heirs on edge. Even the attractive prospect of Firth and Macfadyen squaring off as dueling Darcys is mostly minimized to a few moments of modest voice-raising.

Johnny Flynn, a more recently lusted-over actor from a Jane Austen adaptation (Emma.), floats around the fringes as Mincemeat’s narrator. His initial musing that wartime intelligence is like “a wilderness of mirrors in which the truth is protected by a bodyguard of lies” suggests the character he’s playing, Ian Fleming, probably benefited from some good editors.

There’s a good running gag about aristocrats and officers like Fleming lining up to write novels based on scrapped espionage plots, and although Mincemeat omits mention of Fleming as James Bond’s eventual creator in its where-they-went-next postscript, it leaves room for amusing incidental foreshadowing (such as when Fleming’s eyes alight at the notion of a wristwatch that’s actually a buzzsaw). Fleming’s purple narration lends color to an otherwise pale tale, in which the true story of Mincemeat gets a fictionalized flavor profile of porridge.