Entertaining from start to finish and wonderfully played by a largely female cast, David Arquette has a small role as an escaped convict, Grant’s film beautifully upends the sexist notion that women are naturally inclined to nurture. It surprises, too, as a tribute to the fortitude of working-class women.

This strange and anxious mixture of the Working Women comedies of yesteryear (think: 9 to 5, Baby Boom, and Working Girl) with the cramped hospital horror shows of our Saturday night sleepovers (recall: Visiting Hours, Halloween II, and Dream Warriors) is always compelling, always nerve-wracking, mostly funny, and agreeably gross.

12 Hour Shift doesn’t allow for quite the same kind of bravura showcase that Bettis gave us in “May” — Grant’s film, while plenty deranged in its own right, is nevertheless grounded in reality — but it still depends on the actor’s genius for being loathsome and lovable at the same time.

The material is edgy and at times outrageously gory and chaotic, but Bettis gives Mandy an exhausted, fed-up quality that keeps the movie on track, even (or maybe especially) when she’s pissed off about having to do everything herself.

12 Hour Shift never takes itself seriously enough to make the calamity that ensues anything more than “dumb fun,” and I mean that positively.