On the bare brick wall backing Jen McGinley’s set – half courtroom, half saloon – there is a faded poster of The Searchers, John Ford’s 1956 western. The whisky-swilling hard men who haunt the clubs and dives of The Long Drop may well see themselves as cowboys. They are double dealers and grandstanders, full of bluff and bluster; sometimes cosplay baddies, acting as tough as John Wayne; other times, they are the real thing, meting out beatings and sociopathic violence.

But this is not Monument Valley. Rather, it is the same Gorbals streets outside the theatre where, in a different time, one of Glasgow’s most notorious crimes played out. Adapted with equal parts wit and horror by Linda McLean from the true-life crime novel by Denise Mina, it is the unravelling story of a triple murder.

In 1956, Marion Watt, her daughter, Vivienne, and sister, Margaret Brown, were shot dead as they slept in their beds in the commuter suburb of Burnside, South Lanarkshire. The police suspected Marion’s husband, William Watt, despite his being 90 miles away on the night. He had his alibi, but was shifty with it.

Determined to escape the blame, Watt tracked down Peter Manuel, an underworld figure who claimed he knew the culprit – and had gleaned from him unfeasibly precise details of the crime. Interlaced with court testimony, their night of boozy revelation is as dark as the soot-stained city, a descent that goes from lawless to immoral.

In Dominic Hill’s dreamlike production scenes melt into each other like dissolving fragments of truth. Brian Vernel’s Manuel is all dead-eyed stares and baby-faced charisma. Somehow his self-delusion offsets his nastiness, as if nobody has told him about his own depravity.

As Watt, Keith Fleming is a bumptious businessman, never as in control of events as he would like to appear. Overheated and flustered, he pictures himself among the city’s mercantile elite; the company he keeps – and his endless drinking – shows he is nothing of the kind. He has more cash than class.

Together, they are the odd couple at the centre of a city run by spivvy club owners, sharp-tongued bar staff and bluffing lowlifes. It would be comic were it not so grim, yet it is darkly compelling.

• At Citizens theatre, Glasgow, until 20 June