Writer/director Padraig Reynolds’ nearly single-location slasher Open 24 Hours starts off with a very strong declaration of what this movie is and what it’s about: Vanessa Grasse’s Mary is applying for a job at a crappy, always-open gas station in the middle of nowhere, and the kindly manager looks over her application. She’s an ex-con, and he wants to know what got her in prison. “I set my boyfriend on fire,” she replies evenly. “Did he deserve it?” the manager asks. Mary nods.

Did he ever. There are a lot of complications to Open 24 Hours’ plot, but the upshot is that Mary discovered her boyfriend James was a very prolific serial killer later dubbed The Rain Ripper, and out of fear or perhaps something more sinister, she never reported him. After witnessing the brutal murder of many, many women, she finally tried to set him on fire – but it didn’t take. Now Mary’s out of prison and doing her best to turn her life around, but paranoid delusions, an unwelcoming society, the guilt of her past and just maybe the return of her murderous ex all conspire to thwart Mary’s rehabilitation.

Open 24 Hours uses a mostly effective unreliable narrator approach, as Mary’s first graveyard shift at the Deer Gas Market is beset by flashbacks – or hallucinations? or actual events? – of extreme horror met with moments of intended suspense. We’re never quite sure if The Rain Ripper has actually escaped and is coming after Mary, or if Mary’s just not ready for this abrupt return to society, despite the help of her parole officer Tom (Daniel O’Meara), best friend Debbie (Emily Tennant) and supportive new coworker Bobby (Brendan Fletcher). Open 24 Hours enjoys the rare slasher success of a cast we actually care about, slight but likable characters whom we’d rather not see bludgeoned to death with a sledgehammer (not that we’ll end up having much say in the matter). Grasse is especially valuable as Mary, a mild, squirrelly little thing who manages to engender our sympathy even as we hear how she was the unprotesting witness to dozens of heinous murders.

Really, all of the characters are great, save James, aka The Rain Ripper, whose every moment onscreen is rather eye-rollingly heavy-handed, though not necessarily through the fault of actor Cole Vigue. It’s a sign of another of Open 24 Hours’ weaknesses: every scare is telegraphed, and every emotional beat is megaphoned. This is not a subtle movie, which works when we’re talking gore (because christ, there’s some good gore in this thing), but works less when the film’s trying to build suspense.

Most of the tension relies on the extremely dumb decisions of its characters. Mary seems sensible enough at first, minus that whole serial killer boyfriend thing, but the only times her life seems to be in real danger are when she’s done something extraordinarily foolish to warrant it. She hides behind open cabinets, crawls when she could easily run, never flees out the right door, never takes the car keys, never confides in the right person and hangs up when she finally gets a hold of the cops. It’s maddening, especially by the end, because it’s such an endless litany of boneheaded choices that we kind of can’t blame The Rain Ripper for wanting to rip her. And she’s not the only one! We want to root for the continued survival of grouchy father figure Tom, beer-slamming bestie Debbie and really quite sweet new pal Bobby, but they just keep making James’ job so damn easy.

Still, there’s an ambiguity that works behind some of these dumb decisions: maybe Mary wants to be caught. Maybe all of this is in her head in the first place. We’re never quite sure, and that works fine here. Open 24 Hours will never be an all-timer entry in the slasher genre, but it’s always plenty compelling, with affable characters facing wicked, no-holds-barred violence. Like the Deer Gas Market, it’s nothing fancy, but it’s got most everything you need in a pinch.