Plot taken from Wikipedia:
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.
So Tsutomu is stuck sharing a body with a rather attractive, yet very strong but impulsive, space police agent, while trying to keep his apathetic family and more importantly, his girlfriend, from finding out about his woman problems. In the meantime, to make matters worse, Birdy still has to do her job. Together, they take on a rather secretive group of evil aliens planning to perform maniacal experiments on the unsuspecting inhabitants of Tokyo.
AFAIK this'll be a TV series based on the manga, but will have an original plot.