Natsuka has been in love with Hakushima since high school but is afraid of confessing to him, so he has sex with Seno instead. Then when Seno and Hakushima got into an accident, Seno who woke up first pretended to have swapped body with Hakushima to win Natsuka's love. He succeeded until Hakushima regained consciousness 3 months later. Turns out they were all high school classmates, though Natsuka doesn't remember Seno.
To pretend to be someone else for the one you love is a very sad thing for both of you, because nothing good will come of it when they wake up. This manga is just a fiction after all.
Kakkou no Yume