The film unfolds in three distinct acts, narrated by one of the leading characters, Kari Kamiya, who appears in both the first and third acts. Kari begins by introducing the captivating universe of Digimon, explaining the origin of these creatures, which are formed from computer data and inhabit a parallel dimension known as the Digital World. She elaborates on her role in a select group called the Digidestined, who partner with Digimon to safeguard their world from malevolent forces. Initially, the team consists of just a few members, but soon welcomes a new addition, Willis, who resides in Colorado. Willis is accompanied by his twin Digimon, Gummymon and Kokomon, but he struggles with feelings of isolation and loneliness.
The narrative transitions to 1995, focusing on a fateful night when Kari and her older brother Tai encounter their very first Digimon. In Highton View Terrace, Tokyo, Tai awakens to find Kari intently observing their father's computer as a massive Digiegg appears. The following day, they witness the egg hatch into Botamon, which quickly forms a bond with Kari. While Tai is briefly distracted by a phone call, he returns to find Botamon has evolved into Koromon, a playful, pink Digimon. This transformation, referred to as digivolving, enhances the Digimon's abilities but also disrupts electrical devices nearby.
Kari playfully steals food from their family cat, Miko, inciting its fury, which leads to a comical scuffle among Tai, Koromon, and Miko. At sunset, Koromon surprises them by speaking, declaring them his best friends. Later that night, an electrical surge causes Koromon to digivolve into Agumon, a large, dinosaur-like creature. Kari hops onto Agumon's back as they go on an adventure that leads them to the streets, where Agumon creates chaos by unleashing fireballs.
In a dramatic twist, a second, colossal Digiegg hatches, releasing Parrotmon, a menacing, bird-like Digimon capable of shooting electrical blasts. When Parrotmon buries Agumon under rubble, a desperate Tai encourages Agumon, who digs deep within to digivolve into Greymon and contends with Parrotmon. Although Greymon is struck down, Tai uses Kari's whistle, rousing Greymon to action, who ultimately defeats Parrotmon at the cost of his own existence.
Kari narrates how she, Tai, and six other children were chosen to become the Digidestined based on their witnessing this intense battle. This narration segues into the second act, which is set in 2000, a year after the events from the series Digimon Adventure. Within this backdrop, Koushiro "Izzy" Izumi, a tech-savvy member of the Digidestined, identifies an alarming computer virus invading the internet. Among the worldwide chaos unleashed by this virus, one Digiegg mutates, hatching into a Digimon that greets the world with a cheerful "Hello!".
Meanwhile, Tai grapples with reconciling a dispute with his friend Sora Takenouchi, as he tries to compose an apology letter. Lurking nearby, Kari unwittingly sends the email without Tai's consent. As various montages illustrate the daily lives of the Digidestined, unexpected interactions occur; Joe Kido, the sensible oldest member of the team, misses an important train, while Matt Ishida and his brother T.K. enjoy time in the countryside. Izzy races to Tai's apartment, conveying alarming news about the Digimon, escalating the urgency of their situation.
In Tai's bedroom, they uncover that this digital menace threatens the integrity of technology. They discover that the virus has evolved into a monstrous form, resembling a cycloptic claw, capable of devouring data constantly. Tai longs for Agumon's presence, and just then, Agumon reaches out to Tai via the laptop, revealing he and the other Digimon are aware of the situation. They are joined by Gennai, a digital entity, who offers guidance. Tai attempts to contact the other Digidestined to rally their support, but his efforts are met with obstacles—Joe is too occupied, Matt and T.K. face challenges at home, and Sora avoids direct communication. The confusion among the friends reveals the tensions stemming from their past quarrels.
As the story progresses, the Digimon go on the offensive against the powerful Keramon. A frantic battle unfolds, wherein Greymon and Kabuterimon grapple with the evolved threat of Infermon, a formidable enemy that surfaces as the fights become increasingly intense. A countdown begins; Infermon has hacked critical systems, leading to potential nuclear disaster looming over their heads.
Empowered by teamwork and innovation, Tai and Izzy realize they must act quickly to thwart the deadly countdown initiated by Infermon. Although their Digimon face fierce opposition, they soon amalgamate into a new form, Omnimon, an embodiment of their combined strength. In a tense climax, Omnimon confronts Infermon, racing against time to thwart the missiles launched across the globe.
In the third act, set four years later, the story shifts focus to the next generation of Digidestined. T.K. and Kari are in New York, intent on visiting Mimi, but their journey leads them to realize a new threat looms. The teenage Willis, now grappling with his rogue Digimon counterpart, Kokomon, must decipher cryptic messages as he endeavors to manage the chaos ensuing from his past actions.
The narrative escalates as the Digidestined face challenges in their trek across America. Alliances are formed, battles ensue, and ultimately they discover that Kokomon's essence is tied to the past mistakes of Willis and the ensuing chaos of their digital realm. A heartfelt finale allows the characters to reflect on the enduring bonds of friendship, highlighting the resilience of both humans and their Digimon partners, as well as the promise of renewed connections, even in the face of adversity. The journey concludes with cheer and a touch of nostalgia, underscoring the theme that friendships endure beyond time and circumstance.
