The full plot of Insidious: Chapter 3 runs from a teenager's attempt to reach her dead mother to a rescue inside the Further, the spirit world the series keeps returning to.
After the accident, Quinn is haunted at home by a spirit wearing an oxygen mask, and she recognizes it as the figure she saw in the street before the car hit her. Her father Sean goes to Elise for help. She is grieving her late husband, and whenever she enters the Further she is hunted by a Bride in Black. Her friend Carl convinces her to help.
Alex also calls in Specs and Tucker, two amateur ghost hunters. During their visit Quinn is briefly possessed and lashes out. Elise arrives and works out that the spirit lures people into the Further so it can feed on them. She decides the only way to stop it is to go in herself, and she brings Specs and Tucker with her.
Inside the Further, Elise fights off the Bride in Black and then meets what appears to be her husband, who urges her to take her own life. She sees that it is the breathing spirit in disguise and demands that it free Quinn. Elise cannot beat it alone, and a message from a dead neighbor leads her to call on Lily's spirit. Lily gives Quinn the courage to rip off the creature's mask, and the entity suffocates and is destroyed.
Elise says goodbye to the family and leaves with Specs and Tucker, and the three agree to work together. Back at her home, a final shot of a red faced demon watching her from behind ties the story to the earlier films, as the ending explained breakdown goes on to cover.