Birdy the Mighty: Decode

While pursuing alien criminals named Bacillus and Geega, Birdy and Tute infiltrate Earth under the cover of a rising Japanese idol and her manager. During one night on a routine mission to apprehend the said criminal, a civilian named Tsutomu Senkawa is used as a human shield. When Birdy realized that she killed him in an accident, she decided that the only way to save his life was to fuse her body with his in order to prevent him from dying. Tsutomu thought that it was all a dream until he found that he was unable to control his body one night after he was supposedly dead.

Birdy and Tsutomu try to cope with existing by sharing one body, but leading two separate lives. Birdy's commanding officer, Magius, informs Tsutomu that his original body's being restored and it will be some time before he can live outside of Birdy's body. Bacillus tries to eliminate Birdy by luring her into a movie shooting as cover. Keisuke Muroto tries to take photos of Bacillus and Birdy, but falls unconscious when he was knocked out by debris. A wounded Bacillus took over Geegar's Earth body as his first one was damaged from deterioration and from fighting Birdy.

Bacillus tries to locate and eliminate Birdy, while she, Tsutomu and Tute argue over the constant separate lives that they were trying to hide from the public. Tute gets wounded by Bacillus before he gave Birdy an Elemental Destructor capsule bomb to kill Bacillus and eliminate the threat of him killing an Earth-born civilian in Japan. Birdy mourns Tute's death when he can't heal his wounds from Bacillus' attack.

Birdy returns to her home planet Altaria to attend a trial of one of the smugglers that she had arrested. She and Tsutomu were questioned by a deity named Nejula Gate Altaria regarding Ryunka. Nejula reveals to Tsutomu that Ryunka is used as a nuclear weapon, killing off the inhabitants of a planet named Bilugema. While Birdy visits her hometown, a terrorist bombing destroys a convoy of vehicles.

Birdy fights with King Zero as Nejula corners Gelabu--whom ultimately commits suicide for his "pride" or "honor." Thanks to Birdy's way with words and the simple fact that King Zero's girl can't stand to see lots of people die, she detonates the falling satellite which was targeting the Union. Kashugeza turns out to be an undercover agent under Skeletsu's direction. Birdy is quite frustrated with the fact that Skeletsu tolerated the deaths of her fellow, innocent, Altarians in the terrorist attack in order to get a lead on King Zero.

Sayaka Nakasugi invites her classmates to her mansion for her welcome back party. Birdy investigates the site where she had killed Tsutomu in order to locate a marionette of non-Earth origin. Sayaka's maid Yoshio finds strange things have happened to her, which started to worry Sayaka's grandfather, Katsutoshi. Meanwhile, Nakasugi disappears from the Nakasugi Mansion after encountering a parrot, which was an alien being in disguise.