Ruka to Ita Natsu

A sudden and unexplained incident disrupts Toono-kun's daily life when a flash occurs in the school yard during his sister Yayoi's track and field practice, leaving several people injured. Though Yayoi recovers quickly and is released from the hospital, her behavior changes dramatically. Toono realizes that Yayoi has been replaced by a soldier from the future named Luca, who arrives 31 years ahead of time. He is the only one aware of this substitution, leaving him to deal with the consequences alone.