A group of six friends are heading off on one of those subterranean caving expeditions that just make me crave wide open spaces. The thing is, though, that one of their number takes them to somewhere hitherto unexplored - without telling them - and so once they start to squeeze their bodies and their kit into these unforgiving spaces, they have no idea where they are going nor what to expect. Not long into their expedition, they encounter some cave paintings and signs that perhaps someone had been there before after all, but instead of turning tail and getting back to the surface they press on ...