Part 1 Revelation: Seo-rae did indeed murder her abusive husband, Ki Do-soo. She studied his mountain route, used his phone's health app to track step counts, spoofed her location by visiting a bedridden elder, and pushed Do-soo off the cliff. Hae-jun uncovers this truth after realizing she swapped her client's cell phone. However, deeply in love, Hae-jun covers up her crime, tells her she has "completely shattered" his pride, hands her the incriminating phone, and tells her to sink it deep into the sea.
Part 2 Plot Shift: Hae-jun moves to the foggy seaside town of Ipo with his wife, suffering from depression and severe insomnia. Seo-rae reappears in Ipo, now married to Im Ho-shin, a financial fraudster. When Ho-shin is found stabbed to death in a swimming pool, Hae-jun suspects Seo-rae again. In truth, Seo-rae manipulated the crime indirectly: she knew a victim of Ho-shin's scams (Sa Cheo-seong) wanted Ho-shin dead, so she euthanized Cheo-seong's mother to trigger Cheo-seong's violent rampage against Ho-shin.
The Ending: Seo-rae steals back the recorded phone evidence and leads Hae-jun to a remote beach. While Hae-jun is driving to find her, Seo-rae digs a deep hole in the wet sand at low tide, climbs inside, and covers herself with a plastic bucket. As the high tide rolls in, seawater fills the hole, burying her body and the phone under the sand forever. Hae-jun arrives on the beach, calling out her name frantically, walking directly over the spot where she is buried beneath his feet—forever oblivious to her precise location, doomed to wander the shore endlessly searching for his ultimate unsolved case.