Some years after an unspecified apocalyptic event a chap with crap dreadlocks is living in a shed in Ireland. He spends his days robbing dead bodies, tending to his allotment and wanking into a potplant. Bit like how I imagine retirement is for many.
One day a mother and daughter happen upon his shack and they strike a deal where in exchange for some of his meagre food supplies, can have sex with the daughter rather than the daffodils. Something, something, planting a seed etc.
They don't really trust each other, but manage to form a shaky bond. Other people occasionally turn up and rather than forming further sex pacts, it all gets a bit killy. Then it ends.
It's good but not exceptional, the reviewers, however, seem to be wanking all over it as if it was the worlds sexiest pot of cress.