This is possibly Michael J. Murphy's best known work, certainly in the pre-cert world of video nasties.
It's got a lot to like but at the same time it didn't all hang together that well for me. I appreciate the atmosphere and commend the edginess. The soundtrack alone throws me right back to the VHS era.
A girl turns up to a fancy dress party but is then held captive as part of a bizarre ritual. Simplistic, but there is also a subtle subplot going on if you care to watch the embedded full film below.