Ayoungnewlywedcouplefindthemselvestrappedinanetworkofundergroundcaves.Withherhusbandinjured,thenewbrideisforcedtofindherwaybacktothesurfaceinordertosavehislife.Intheprocess,theydiscoveraterrifyingtruthaboutthe"underneath",theyarenotalone.Somethingevilishuntingthem.