Storyline with major spoilers:
Oh Ha-rin (Yeri) did not like celebrate her birthday since her high school crush Ji Seo-joon (Yang Hong-seok) died mysteriously on her birthday 10 years ago. They had agreed to meet at the photography club on her birthday, but Seo-joon was found dead there with his wrist cut, presuming to have committed suicide. For the 10 years, Ha-rin could not forget Seo-joon. She then came across some photographs that Seo-joon had taken with his film camera 10 years ago. Unexpectedly, Ha-rin found that she could return to the date on the photograph if she burnt that photograph. Ha-rin then decided to return to the past to save Seo-joon. Initially, she thought Seo-joon had committed suicide due to family problem as Seo-joon was adopted by his foster parents whose birth daughter Ji Hye-min (Kim Yi-seo) went missing when young. Hye-min was recently found, and Seo-joon seemed to be upset that he became more distant from his foster mother since the appearance of Hye-min.
Ha-rin managed to save Seo-joon on her 3rd attempt but Hye-min was found to meet with a fatal accident instead, so Ha-rin returned to the past to save Hye-min, only to find Seo-joon to still die on her birthday. Ha-rin then found out that Seo-joon could not have committed suicide as he had injured his left hand (and therefore could not slash his right wrist), and he was administered with an animal tranquilizer. It was later revealed that Hye-min had murdered Seo-joon out of jealousy, and there was an eyewitness who saw Hye-min committing murder. Present-day Hye-min learnt about the secret of time travelling through burning the photograph. Hye-min then returned to the past to kill the eyewitness first, before attempting to kill Seo-joon on Ha-rin’s birthday. Ha-rin and Hye-min then fell off the school building in Ha-rin’s attempt to stop Hye-min from hurting Seo-joon. Seo-joon found the last photograph to return to Ha-rin’s birthday to save Ha-rin, as well as stopping Hye-min from committing another murder. Seo-joon survived Ha-rin’s birthday in the end, and became a professional photographer, while Ha-rin went through her 10 years of life again, this time with Seo-joon still alive.
This web drama consists of 16 episodes with 30 minutes per episodes, so it was a really short and easy to watch drama. The time travel was also easy to follow, with the rules for time travelling being highlighted at the end of each episode. The only thing I did not like about the show is probably Hye-min and her obsession in possession and revenge, which could seem quite annoying and scary to watch such crazy character.