Blue Birthday

The series opens with Oh Ha-rin witnessing her best friend and first love, Ji Seo-joon, lying dead in a pool of blood in the photography studio of their school on her 18th birthday. Ten years later, Oh Ha-rin now works at an animal protection center, and gets a task to rescue an animal from her previous high school. She can get the job done despite traumatizing memories. While leaving from work, Ha-rin comes across an old photo shop and notices Seo-joon's camera on display, buying the camera and some old photographs from the mysterious shop owner, who warns her not to get stuck in the past. A confused Ha-rin goes to her birthday party organized by her friends and leaves after one of them, Kim Ui-young, mentions the unexpected and mysterious suicide of Seo-joon. After going through all the photos, Ha-rin decides to burn them, and throws one of the nine photos into the fire, only to wake up as a high school student in 2011. Seeing Seo-joon alive and on his bike, she pretends it to be a dream and runs towards her lost friend. She trips on the stairs but is able to catch up with him and hug him, which is caught by her now younger friends. Throughout the day, Ha-rin acts weirdly and follows Seo-joon, but when the day ends, she returns to reality. This is when she realizes that burning one photo from the bundle can transport the person back to the same day the photo was taken. The next time she time travels, she finds herself watching Seo-jun's PE class where he hurts his left hand, and recalls the police information that mentioned he had cut his artery in his right hand. However, this would be impossible as Seo-jun was left-handed. When she sees him drinking soda, she realizes that Seo-jun didn't kill himself but was actually murdered. She tells him out loud, but he brushes it off. With the help of Eun-song, Ha-rin can get Kim Shin-woo arrested, only to find out that he is a witness and that the murderer is actually a female. It is then revealed that Ji Hye-min is the real killer and, with her aware of the secret of traveling back to the past, she steals all the pictures from Ha-rin's house and burns them to travel back in time to murder Shin-woo. When a video recorder is found, the group goes through the video and sees a girl wearing the same hairpin and jacket as their friend Do Soo-jin. Thus they start suspecting her of murdering Shin-woo. However, she has an alibi which is revealed at the police station. Soo-jin was suffering from constipation and went home by asking for a lift from a delivery person of a Chinese eatery, but could not remember the name. The group hears her alibi and decides to save her. Seo-joon and Ha-rin then find another camera whose contents reveal that Hye-min is the real killer, and that she disguised herself as Soo-jin to murder Shin-woo. Hye-min had a troubled past and therefore had developed psychopathic tendencies. She believed that Seo-jun was only a substitute for her, and now that she had returned, he should die for living her life until now. While being taken away to prison by police officers, Hye-min tries to run away, but dies in a car accident. In the end, Ha-rin and Seo-jun live happily together.