What surely would have made a good reality-TV series — three juvenile delinquents from the big city, plus an awkward kid with no friends, are dropped in the Scottish Highlands and left to find their way back to civilization — works even better as a dark comedy goof when a couple of lunatics start shooting at them from afar. The stakes are high, but so are half the characters in Ninian Doff’s irreverent survivalist satire, which makes this deranged camping trip — with its phallocentric hip-hop jams, improvised pyrotechnics and hallucinogenic rabbit droppings — all the more unforgettable.