Kay is a quiet and withdrawn woman who leads a relatively uneventful life. She works at a nursery and maintains a cautious relationship with her boyfriend Louis. Her world is disrupted when her sister Dawn, nicknamed "Sweetie," re-enters her life. Sweetie is a wild, unpredictable, and mentally unstable individual who often clashes with her family. This familial tension is heightened by Sweetie's bizarre behavior and erratic mood swings, which cause significant stress for everyone involved.

The already strained familial bonds further unravel as Sweetie's presence brings to light various underlying issues within the family. Their father gets involved, attempting to mediate the growing conflict but often exacerbating it instead. The story takes the audience through a journey of emotional turmoil, deep-rooted family secrets, and the complexities of sibling relationships. As the dynamic between Kay and Sweetie shifts, their family must confront past grievances and the fragility of their connections.