Contains spoilers for this episode and earlier episodes.
Emily's friend Amanda asks for help with her father Wayne, who has been confined to a psychiatric hospital. Lightman goes inside to discover whether Wayne needs that confinement, ignoring Gillian's wish to approach the case together. His behaviour gets him held for evaluation, and the medication and food he receives leave him hallucinating. He sees his dead mother and confronts his abusive father while the team tries to work out what is happening to him.
Emily worries when Cal does not return home, and Gillian recognises that his approaching birthday has unsettled him. He is about to become older than his mother was when she died. The investigation then turns towards muffins Amanda brings Wayne. Testing reveals a drugged delivery system rather than evidence that Amanda has deliberately harmed her father. Her aunt Gina has been altering the muffins to keep Wayne confused and away from their estate and its horses.
Cal stages an interview with Wayne apparently recovered, forcing Gina to confront what she has done. Her interest in controlling the property explains the repeated doses and the manufactured appearance of continuing illness. Wayne and Amanda are reunited. Cal returns home and talks with Emily about seeing her grandmother, accepting that his own frightening experience may have helped him face memories he usually avoids. Their conversation ends around his birthday breakfast, with Emily still challenging the risks he took to investigate her friend's problem.