The protagonist Tomochika Tatsuya helps secretly the "story's protagonists" around him, like the "friends characters from bishoujo games or light novels".Tomochika have the "rules" that god imposed to him since his childhood:
"In differents groups, everyone but me is a "protagonist" and his "heroines"
"I won't be a lover candidate because the "heroines" will love the "protagonist""
"I must help the "protagonist" to get engaged with a "heroine"

And like always doing his "friend" role at the Valentine Day in Yahagisakura School 2nd year, a mysterious girl who called herself a "support character" appears in front of Tatsuya when he remembers the sadness from a role that only gives loss.
"I'll grant one wish of yours"
"...at least one time I want to become a "protagonist" and have a marvelous love"
"So let's steal the "heroine" from the "protagonist"
With the mysterious girl Kusumoto Mikoto, the "friend"'s revenge drama starts....?

[From The Official site]