It’s essential to acknowledge the films that will never see the light of day: the unfunded epics, the unmarketable art films, and even the unengaging streaming products. Filmmaker Charles Shackleton experienced this loss firsthand when his meticulously researched documentary on the Zodiac killer case was abruptly halted after years of development. However, Shackleton’s vision was too expansive to remain unrealized. In the humorously titled “Zodiac Killer Project,” he shares how his film would have unfolded, offering a reflective and candid confession.

The honest narrator, also a published film critic, brings more than just his regrets to the table. This documentary serves as a playful critique of the true-crime content bubble and the genre’s tendency to copycat itself.

Although Shackleton’s feature cannot legally reference its source material, the 2012 book “The Zodiac Killer Cover-up: The Silent Badge,” it functions almost as an official adaptation. The director had initially secured film rights from the family of its author, former CHP officer Lyndon E. Lafferty, who passed away in 2016. Lafferty’s book was the result of a decades-long pursuit, driven by his belief that he had encountered the Zodiac killer and subsequently launched an unofficial investigation after being reprimanded by his superiors. Shackleton was drawn to Lafferty’s work as it offered new insights into the extensively examined Zodiac files. Unfortunately, while scouting locations in the killer’s former haunts, the rights agreement with Lafferty’s family fell apart.

In “Zodiac Killer Project,” Shackleton recounts the narrative arcs he had envisioned for the film, speaking in a meandering, contemplative tone over sweeping shots of the Bay Area. Occasionally, he drifts into a trance, imagining climactic scenes, only to break the spell with laughter. With insert shots of crime-scene flashbulbs and burning documents—visual clichés that Shackleton humorously critiques—“Zodiac Killer Project” closely resembles a true-crime documentary. The central irony is that it achieves this using only spare parts. However, this also highlights the limitations of the thought experiment: a parody of a monotonous genre is still bound by its conventions.

Shackleton, appearing in a few recording booth segments, expresses genuine disappointment over his original ideas. “Fuck, it would’ve been good,” he sighs after outlining his captivating opening. Yet, the more complex and engaging aspect of “Zodiac Killer Project” is evaluating how seriously Shackleton was ever invested in this material.

Viewed generously, Lafferty was a man who believed he could uncover the truth by taking matters into his own hands. However, objectively, his pursuit was a wild goose chase: a guerrilla operation limited by his fixation on a single suspect. The narrative becomes increasingly absurd, even featuring a twist where Lafferty and his suspect cross paths in the same social circles later in life. Robert Graysmith, the obsessive author of the most well-known book on the case, who was portrayed by Jake Gyllenhaal in David Fincher’s “Zodiac,” would have also been a character in Shackleton’s film. In this context, Graysmith represents the success story compared to Lafferty’s mundane efforts, akin to how Bob Dylan plays into the ironic conclusion of the Coen brothers’ “Inside Llewyn Davis.”

Even more amusingly, Shackleton’s proposed conclusion for his project closely mirrors the penultimate scene of Fincher’s film. While these similarities go unacknowledged, it serves as another instance of Shackleton highlighting true-crime tropes, particularly the cynical nature of their ambiguous endings. He offers numerous critiques of the genre that has proliferated in the last decade, including the diminishing restraint of “Making a Murderer” and the hypocritical moralizing found in Ryan Murphy’s Dahmer series “Monster.” At one point, he humorously notes how many former police officers refer to themselves as “bulldogs” in interviews, pointing out the genre’s capacity to operate with “no direction required.”

The uncomfortable reality is that the original, unmade Zodiac killer project—not “Zodiac Killer Project” itself—seems beneath an artist of Shackleton’s caliber. The British filmmaker first gained recognition in 2016 with his 10-hour film “Paint Drying,” created to protest censorship and the high costs of independent filmmaking, which compelled the British Board of Film Classification to watch exactly what was submitted. Yet, work is still work, and Shackleton acknowledges early in the documentary that many nonfiction filmmakers find themselves drawn into the true-crime genre in today’s market.

The director takes pride in the craft of his original vision, but his ability to vividly articulate a film that doesn’t exist underscores the genre’s almost automated organization in its rise to market dominance. The dry, minimalist “Zodiac Killer Project” was selected for Sundance’s experimental NEXT category, a curatorial achievement that feels like an ambush in itself. Shackleton candidly questions, “How many people are even going to watch this realistically?” Certainly, far fewer than the audience that tuned into Netflix’s own Zodiac killer docuseries while multitasking last year.

Addressing viewers who understand the limitations of what films can be made, “Zodiac Killer Project” serves as a sharp commentary on how many artists have been funneled into a creative dead-end by a trend-driven market.