In 1989, the high-flying FBI officer Mark Putnam became a gruesome footnote in bureau history by killing the informant with whom he was having an affair: a young woman called Susan Smith. That made her the first murder victim of an FBI agent.

It is a bizarre case of corruption and misogyny, and it deserves a substantial movie. Instead, we have this moderate one: a brash, true-crime noir melodrama from screenwriter Chris Gerolmo, who gives Smith a narrative voiceover from beyond the grave, with Phillip Noyce directing.

Emilia Clarke does a good job as Smith, the troubled, lonely young woman in a depressed former mining town in Kentucky who is addicted to drugs and has been abused by men all her life: she still lives with her ex-partner, small-time coke dealer Cash (Johnny Knoxville). Jack Huston is the straight-arrow FBI man Putman, married with a baby, who arrests Susan and Cash, and with a lawman’s natural ruthlessness senses that poor, love-starved Susan might turn snitch and give him serious information about drug crime and robberies, in return for tender attention and money. He also gets turned on by the sexy danger that Susan represents; soon they are having a torrid relationship, and Mark’s wife, Kathy, (Sophie Lowe) turns a blind eye.

The stranger-than-fiction weirdness and emotional dysfunction are what’s interesting here, and the film doesn’t quite take the lid off it. But Clarke is effective as Smith and carries the picture. Huston is also plausible as the uptight G-man who is smugly pleased with the dangerous game he’s playing – manipulating Smith while getting a sleazy sexual bonus as well as a career uptick. He becomes just another of the ugly male exploiters. There’s an interesting cameo from Thora Birch (from American Beauty) playing Sarah’s long-suffering sister Jolene.

• Above Suspicion is available on digital platforms from 13 July.