I was so confused by the structure of this story. Lemme see if I got this correct:
So it starts off with a flashback to a group therapy session with Hirasaka Koushirou and Okitsu Ryou. Koushirou was saying that he became a shut-in after thinking that he hallucinated an imaginary friend named Yuu-chan being kidnapped by a doctor he saw afterwards, but then came out of seclusion once he found proof that his friend was real. He was going to various hospitals to look for the doctor and find out why he took Yuu-chan away (Why didn't he just ask his parents what doctor he saw when he was younger?). At this group session, he met Okitsu Ryou, who knows that the doctor he was looking for already died. Ryou tells Koushiro to get into a certain university to meet "Yuuto."
Back to present day, Koushiro is in a club with "Okitsu Yuuto" who is calling all of Okitsu Ryou's classmates in high school to see if they remember him (I didn't get the purpose of this part. Ryou has claustrophobia which made it hard for him to attend class, but why did it matter to Yuuto and Ryou that someone remembered him in high school?). Koushirou tells his story to Yuuto and the other guy Minami (Why was he in the club as well? What was the purpose of the club?) which makes Yuuto realize that he is the Yuu-chan in Koushirou's memories, even though he got his own memories of that time sealed away by his grandfather, who was the doctor. Turns out Yuuto had already been kidnapped by his father, who possibly was dead the whole time Yuuto met with Koushirou in their childhood.
His grandfather found him and adjusted his memories so that he didn't remember that 6-month period in that neighbourhood. His friend Ryou, who was one of the grandpa's patients, let Yuuto use his last name "Okitsu" to attend school in his place since Yuuto forgot that his dad was dead and was living in hiding from him (I didn't get this part either. Why couldn't they at least tell him that his dad already died so he didn't have to grow up in fear and hiding?). Yuuto only remembers his childhood friend Shirou-kun because Ryou wrote down all the stories Yuuto told him in a notebook.
There's also a part where Koushirou says Yuuto blew him?? was this in the past or present? Was it an implication that Yuuto's father sexually abused him? still so confused.