"AniDB":

While chasing down a monster on Earth, intergalactic law enforcer Birdy ran into a young man named Tsutomu. Unfortunately, he was killed during the incident. Birdy"s superior tells her that she must bond with Tsutomu in order for him to live out his life. So Birdy and Tsutomu become one. However, Birdy still has to stop the bad guys.

"AniDB":

Based on a shounen manga by Yuuki Masami, serialised in Shounen Sunday.

Tsutomu Senkawa is an average high school kid who is busy studying for his senior entrance exams. Tsutomu goes out for some fresh air, after a hard day"s work studying for a big test, when he runs into a guy fleeing from a pretty young lady. The pursued man is actually an interplanetary criminal on the run from a Federation agent called Birdy Cephon Altera. Caught in the midst of this chase, Tsutomu gets thrown by the man at Birdy, which kills Tsutomu. Fortunately for Tsutomu, there is a way to keep him alive, although it comes at the price of discarding his body. He ends up merged into Birdy"s body.