The Core Revelation of the Fire: Throughout the film, Seo-yeon harbors deep hatred for her mother, believing her mother left the gas stove on, causing the explosion that killed her father. In the past, Young-sook discovers that young Seo-yeon herself was playing with the stove knobs and caused the fire. Seo-yeon’s entire adult identity had been built on a false accusation born of childhood denial.
The Escalating Bloodbath: After Seo-yeon warns Young-sook that her stepmother plans to murder her during an exorcism, Young-sook overpowers and kills her stepmother. Unshackled, Young-sook's psychopathy emerges. When local strawberry farmer Seong-ho drops by the house in 1999 and notices blood, Young-sook murders him. As a consequence, in 2019, Seong-ho vanishes mid-sentence.
Hostage Warfare Across Time: Young-sook murders Seo-yeon’s father in 1999 when he visits the house, erasing him instantly from Seo-yeon’s 2019 reality. Young-sook holds 8-year-old Seo-yeon hostage in 1999, torturing her with scalding water to force 2019 Seo-yeon into finding out the police evidence that led to Young-sook's arrest in the original timeline.
The Climax & The Mid-Credits Stinger: In 1999, Seo-yeon’s mother arrives at the house with a police officer. Young-sook kills the officer, but Seo-yeon’s mother tackles Young-sook over an upper-floor railing, causing both women to crash down together. In 2019, Seo-yeon finds herself reunited with her scarred but living mother by her father’s grave. However, the mid-credits scene reveals that 2019 Young-sook had called 1999 Young-sook just before the confrontation, warning her not to let go of the phone. 1999 Young-sook survives the fall and murders Seo-yeon’s mother after all. The mother instantly vanishes from Seo-yeon’s side in 2019, and the film cuts to adult Seo-yeon bound to a chair in Young-sook’s dark, bloody basement, sealed in complete torment.