Six-year-old Lily Newman accompanies her mother, Charlotte, on a late-summer visit to her Aunt Beth's sheep farm in the middle of wild Welsh countryside. Beth, whom Charlotte considers to have greatly disappointed their parents by marrying a humble carpenter, now lives with her son and triplet daughters, her husband having died two years earlier. While staying on the farm, Lily becomes fixated with a fairy tale her cousins make up about a Mushroom...