Some teenage metalheads find some ancient sheet music and play it thinking it's a lost track from one of their favourite bands.
Turns out it's the 'Dark Hymn', a piece of music that, when played, summons the king of demons and results in everyone within earshot vomiting blood and becoming possessed. Pretty much Kiss FM but slightly easier on the ears.
Anyway, after dropping such a sizable bollock, the chums have to battle through town and confront evil whilst trying to undo the mayhem they've created.
Although similar in plot to a number of films I've seen, this one benefitted from being a comedy with some actually funny jokes, the scene early in the film featuring evil goons being berated for a botched decapitation in particular made me do a plump LOL.