Tsutomu, an average middle school kid busy studying for the final exams to enter high school runs into a man running from someone and gets caught up in the chase. In all the confusion he gets accidentally killed. Luckily Birdy (an interplanetary agent) knows a way to save him, unfortunately that means joining bodies to become one. So now he is stuck with this officer and along for the ride capturing criminals and saving lives.

[Source: Anime News Network]

Federation investigator Birdy chases the space criminal Geega through space to Earth, where he hides in the entertainment industry under a human disguise. Birdy goes undercover herself, posing as famous model “Arita Shion” that often uses a ‘space girl’ look. A male student, Senkawa, stumbles upon a fight between the two one night in an abandoned building only to be severed in half as he is used as a shield by Geega against Birdy. Birdy heals him from imminent death in some fashion- but Senkawas body, and life, are now changed and permanently linked to Birdy.

[Source: Anime News Network]

Birdy and Tsutomu are still co-existing in the same body, as Birdy continues her duties as Federation Investigator and celebrity persona, and Tsutomu seeks to balance them with his normal everyday life. When a criminal group connected to the Ryunka incident escape from custody, hiding themselves and their alien identities on Earth, Birdy and Tsutomu are thrown back into action. It seems, however, that there are other forces involved, as well as old faces, which give Birdy time to reflect on the past, and the people that influenced her.

[Source: Anime News Network]

This OVA connects the first and second Birdy the Mighty Decode seasons.