"Never make a promise at a funeral. It's my new motto."

When Agnes made a promise to her girlfriend, Ellie, she thought they would be together for ever. But when she has to keep that promise a year later, it puts Agnes in a situation she could never have predicted - climbing a desolate mountain in miserable weather, with Ellie and her new boyfriend, Steve. And when the weather takes a threatening turn and the tension between the trio hits its peak, Agnes will have to push herself further than she ever thought was possible ...