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Under the Banner of Heaven has been a long-gestating project for screenwriter Dustin Lance Black, who spent years developing it as a feature before expanding it into a seven-part miniseries. It’s easy to imagine that abandoned film version zeroing in on the investigation into the brutal 1984 killing of Brenda Wright Lafferty and her 15-month-old daughter, but Jon Krakauer’s 2003 nonfiction account of the crime dug deeper to explore the often-violent history of Mormonism, and Black’s attempt to include that historical context in his adaptation has left his series feeling distended and unbalanced.

Black – who was raised in the Mormon faith – structures his story through flashbacks that introduce us to the vast and influential Lafferty family, referred to as the “Mormon Kennedys”, and to the wider practices of the Church of Jesus Christ of Latter-Day Saints (LDS for short). The most effective material focuses on the Church’s misogynistic power structure, where LDS men are the “priesthood holders” while a woman’s role is to produce children and obey her husband. The ill-fated Brenda (Daisy Edgar-Jones) is more worldly and independent than the other Lafferty wives, and she immediately raises eyebrows with off-colour remarks and her willingness to question the standard way of doing things. “Mind your property,” the family’s patriarch (Christopher Heyerdahl) sternly warns her husband.

Black has created two fictional detectives to lead this true-crime story. Jeb Pyre (Andrew Garfield) is a devout Mormon who has never doubted the teachings of his church, while his partner Bill (Gil Birmingham) – a Paiute, and the only non-white member of this community – presents a more cynical outsider’s view. Watching the relationship evolve between these two excellent actors is one of the most rewarding aspects of Under the Banner of Heaven, and their cultural perspectives freshen up a show that can be tiresomely generic in its approach, with its tremulous handheld camerawork and murky palette.

There’s more than enough material here for a miniseries, but it’s the introduction of a third timeline that really causes problems. Every time a character evokes an incident from the life of Mormonism founder Joseph Smith, it leads to an unnecessary historical re-enactment of said incident. Black tries to draw direct parallels between the Lafferty case and the origins of Mormonism, but the 19th-century scenes look cheap and unconvincing, and they bring nothing to the central drama besides interrupting the narrative momentum. By the time we reach the incoherent cross-cutting at the climax of episode five (the final episode made available to critics), the loss of a more focused feature film version of this tale feels regrettable.

► Under the Banner of Heaven is available to stream on Disney+ now.