"Birdy the Mighty Decode: The Cipher" is a two-episode OVA that serves as a sequel to the "Birdy the Mighty Decode" series. The story continues the adventures of Birdy Cephon Altera, a galactic officer from the Federation, and Tsutomu Senkawa, a high school student whose body she accidentally destroyed and now shares with him. The narrative is set in a futuristic world where Earth coexists with advanced alien civilizations, and Birdy is tasked with maintaining intergalactic peace.

The plot revolves around a mysterious cipher that begins appearing across the city, causing widespread panic and confusion. This cipher is linked to a series of strange phenomena, including the sudden appearance of powerful, otherworldly creatures. Birdy, alongside her partner Nataru, investigates the source of these disturbances, uncovering a conspiracy that threatens both Earth and the broader galaxy. The cipher is revealed to be a form of alien communication, tied to an ancient artifact with immense destructive potential.

As the investigation deepens, Tsutomu becomes increasingly involved, using his unique perspective and connection to Birdy to aid in solving the mystery. The story explores themes of trust, cooperation, and the consequences of advanced technology falling into the wrong hands. The relationship between Birdy and Tsutomu is further developed, highlighting their growing bond and mutual reliance despite their vastly different backgrounds.

The OVA maintains the series' blend of action, science fiction, and character-driven drama, with a focus on unraveling the cipher's secrets and preventing a catastrophic event. The narrative is tightly paced, delivering a self-contained story that expands on the established universe while providing closure to some lingering questions from the main series. The main characters, including Birdy, Tsutomu, and Nataru, are central to the plot, with their interactions and decisions driving the story forward. The OVA also introduces new characters and challenges, adding depth to the overarching narrative and setting the stage for potential future developments.