Space Federation investigator Birdy Cephon Altera accidentally kills high schooler Tsutomu Senkawa while chasing an alien criminal on Earth, then saves him the only way she can, by merging their bodies into one. The two have to share a body and a daily routine, trading control between Tsutomu's school life and Birdy's actual job: tracking the terrorist Christella Revi through Federation politics and alien crime rings that keep spilling into his ordinary world.