How far would you go to fulfil a promise to your oldest friend? Nina, in the final days of her illness, asks her best friend Marie to look after her family when she's gone. And Marie does everything she can to step into Nina's shoes after her friend's tragic death. She tries to do the best by Nina, tries to be there for Nina's young children, tries to support Nina's grieving husband, tries to keep their lives on track despite their terrible loss. But when Marie discovers a dark secret that Nina had kept concealed, she is forced to confront the past, and those shared secrets come back to haunt her too.