The main character, Ichinose has a crush in elementary school, the blonde girl Fumika. However there's another girl who has a crush on Ichinose, her name is Midou. He rejects Midou, but gives her false hope by saying that probably if Fumika didn't exist, he might have liked her. This prompts Midou to kill Fumika, which Ichinose witnisses the aftermath of. Midou gets caught and put into some youth detention facility.
Skip forward to high school, our main character Ichinose is still quite traumatized by the events of elementary school, he is afraid to make friends because he feels that if he gets close to anyone they'll get hurt, just like Fumika got hurt. Its mostly just a psychological thing, he has never seen Midou since that one incident, but he's still very afraid. He has one friend, mostly because that guy seems to be a popular dude who has many friends. And there's this one girl called Yotsuya, who really really REALLY likes Ichinose.
After a while Ichinose lets his guard down, and decides to get closer to Yotsuya, and he begins to feel safer again and has a good time with Yotsuya, who is kind of healing his traumatised heart. However just as this is happening Midou returns from her time 'away' and kidnaps Yotsuya. Midou when meeting with Ichinose lets him know that twice now, he has forgotten about how she was the "first" and we the readers have no idea what this means, it's hinted that Midou knows Ichinose from before even the first chapter but Ichinose forgot.
That's pretty much it so far, obviously lots of little details left out, but the main focus of the story so far seems to be how Ichinose's character is being warped and traumatized by Midou's existence, and how he is spiraling further and further into despair because of it. Only time will tell us where the manga author will take the story next.