Fifteen years after leaving Beacon Hills, Scott now runs an animal shelter in Los Angeles, adjoined to Deaton's new clinic, and is single. Lydia works at an energy company in San Francisco and has broken up with Stiles after receiving a recurring vision of his death. Derek has a 15-year-old son named Eli, and runs an auto repair shop with Malia and Peter. Mason has joined the Beacon Hills police force. Liam lives in Japan and works at a restaurant with Hikari Zhang, a kitsune, where they guard the urn containing the Nogitsune. A hooded figure attacks the restaurant and frees the Nogitsune.The figure sets numerous fires in the Beacon Hills forest, while Scott, Lydia and Chris receive visions of Allison, who had died by the Oni's hand. They theorize her soul is trapped in bardo and cannot move on until they perform a ritual. They return to Beacon Hills along with Jackson for the ritual at the Nemeton, which unexpectedly resurrects Allison. She awakens with amnesia, not recognizing Scott and retaining only fragmented memories of her family's feud with Derek. She attacks Scott and flees the hospital.Deaton realizes the Nogitsune has been possessing Chris and causing the visions of Allison in order to trick Scott's pack into bringing her back from the dead. The Nogitsune uses the Nemeton's power to physically manifest and creates nine Oni. It approaches Allison, claiming that Scott destroyed her family. Allison attacks Derek and hunts Scott and Eli until Scott lets her stab him with a wolfsbane-coated dagger. She takes him to the town's stadium to use him as bait for the rest of his pack. He manages to trigger more of her memories, and she forms a truce with him against the Nogitsune.The Nogitsune abducts Liam, Hikari, Derek, Eli, Noah, Mason and Deaton, and holds them hostage in an illusionary bardo. Lydia and Jackson examine the forest fires and realize that rowan trees were burned in order to produce mountain ash. The culprit is Adrian Harris, who has been in hiding since his apparent murder, blaming Scott's pack for his misfortunes and plotting revenge. Harris surrounds the stadium with mountain ash and forces Lydia to watch her friends' plight so the Nogitsune can feed on her pain. Lydia produces a banshee scream that causes Allison to remember her past upon hearing it. Scott persuades the Nogitsune to let everyone go if Allison executes him. She reluctantly shoots him, but Hikari's kitsune spirit shields him from harm. Parrish burns through the mountain ash, and he, Chris, Melissa, and the others join the pack in bardo. The reunited pack kill the Oni while Scott, Derek and Eli overpower the Nogitsune. Derek restrains it until Parrish incinerates them both. Derek's eyes turn red in his final moments, becoming a true Alpha through his act of self-sacrifice. Allison reunites with Scott and her friends.At Derek's funeral, Noah bequeaths ownership of Stiles' Jeep to Eli. Scott resumes his relationship with Allison and plans to eventually adopt Eli. Parrish has Harris committed to Eichen House. (thanks to Wikipedia)