From fansite( http://tetsuwan_birdy.tripod.com/synopsis.htm ):

“Meet Tsutomu & Birdy - a boy and a girl from opposite ends of the galaxy with absolutely nothing in common. Tsutomu was an ordinary high school student. Birdy’s a maverick Space Federation Officer armed with superhuman strength and - as Tsutomu soon discovers- a devastating energy blast.

Now they have one thing in common: Their body. While apprehending an alien felon hiding out on Earth. Birdy accidentally blasted Tsutomu- killing him instantly. Now, Tsutomu must “time-share” his body with Birdy. And when she isn’t yanking him around like a marionette, he’s transforming into a girl in full view of his friends and family!