Nobody Knows (tv-series, 2020)

Choi Su Jung, a 16-year-old high school student, is brutally murdered by a serial killer. Her friend Young Jin gets to take a phone call from the killer and says that she’ll find and kill him. Over two decades later, Young Jin has become a police officer and still desperately tries to find the serial killer. She has a young, middle-school-student neighbor Eun Ho. He gets shocked by what’s inside Young Jin’s room when he comes over to her place.