This is all alongside Tsutomu's growing feelings for a girl in his class, Sayaka, whose wealth and upbringing have only left her lonely and isolated, and who hungers for friendship and connection, however she can find it. Birdy's quest for the Ryunka potentially leads her into the same social circles as a Bill Gates-like tech genius who may have gotten to it first. The first half of the show deals with Birdy and Tsutomu trying to track down a kind of alien superweapon, the "Ryunka", not just because of how destructive it is but because it can be fused with a living host and thus made almost undetectable.
These two have to learn to share the same body responsibly long enough to figure out how to untangle themselves, including problems like embarrassing bodily functions and how to deal with Tsutomu getting no sleep and missing class whenever Birdy takes off after her enemies. And for a little while, it seems like Birdy may follow the same basic pattern. This is a fun premise, if not a totally original one: e.g., the riotous body-sharing comedy All Of Me, where Lily Tomlin and Steve Martin have to pull this same stunt. It's either Tsutomu "piloting" the body with Birdy inside riding shotgun, or Birdy in charge, leaping across rooftops while Tsutomu inside looks on haplessly. But the only way to save Tsutomu's mind was to temporarily stick it in Birdy's body - and now only one of the two can incarnate at any given time.
Apparently there was enough of Tsutomu's body left to salvage and stick in a stasis chamber until the genetic engineers of Birdy's civilization can figure out how to reboot him. Tsutomu steps in at the wrong moment, and Birdy, unable to pull a punch fast enough, accidentally kills him. One night he's exploring a derelict where Birdy, now on Earth, has cornered a most-wanted cosmic criminal. Just the two of usīirdy opens with two characters: the titular Birdy Cephon Altira, a sassy space cop in the "disintegrate first and ask questions never" mold and Tsutomu, a genial high school kid whose idea of pushing the envelope is to sneak into abandoned buildings on urban spelunking missions. Shogakukan / PROJECT BIRDY A law enforcement accident leaves human Tsutomu and alien copy Birdy sharing a body.It's a great example of how science fiction and social comedy can fuse without compromising either end of the deal. Then it spirals out wider and wider until the original premise all but vanishes, replaced by something far more ambitious, far more fun, and far more intelligent. Birdy The Mighty: Decode, vastly expanded from an earlier OVA adaptation of a manga, begins with a premise that would be fun even if it were used for nothing more than a goofy comedy.
Few things exhilarate like an intriguing idea that's just the starting point for one great leap after another. Few things disappoint like an intriguing idea degenerating into a missed opportunity.