In Thailand, Danish businessman Phillip Eppermint arrives to inspect his new factory. After a boy named Quang accidentally uses Eppermint’s scarf to fashion a checkered ninja plush, Eppermint brutally beats him to death. At that moment, a bolt of lightning strikes, and the doll becomes possessed by the spirit of a deceased ninja. The Checkered Ninja ends up on a Danish cargo ship, where he is discovered by Stewart Stardust, who passes the doll to his nephew Alex Stenstrom as a birthday present.
At school, the ninja steps in to defend Alex’s friend Odysseus from a bully named Glenn, using Alex’s own voice to carry out the rescue. After school, Alex discovers the doll is alive when he insults Glenn, and Glenn chases him through Albertslund. The ninja swiftly removes Glenn from the pursuit, and the two quickly become friends. The ninja makes a deal with Alex: he will help him, in exchange for Alex unknowingly aiding the ninja in exacting revenge on Eppermint by using a ruse involving Eppermint’s lost credit card. As the plan unfolds, Alex realizes that Eppermint shares the same last name as his crush, Jessica, and, with a tip from his mother, learns that Jessica’s father is also named Phillip. To keep Alex close to Jessica, the ninja ensures he is invited to Jessica’s upcoming party on Saturday.
Things take a wild turn when Alex’s stepbrother Sean, in a bid to reveal his own crush on Jessica, chips in via the school intercom. The ninja, again borrowing Alex’s voice, confesses Alex’s love for Jessica to the entire school. The pair then set their sights on a duel with Glenn on the day of the party, choosing the playground as their battlefield and laying clever traps to guarantee Alex’s victory. The night before the fight, the ninja shares a legend about ninja clans in Iga Province, where samurai massacres left a protector named Taiko Nakamura to safeguard the clan’s children. Betrayal by a clan member led to a devastating attack by Nobunaga’s soldiers, the burning of Nakamura’s hut, and the deaths of the clan’s children. Enraged, Nakamura vowed vengeance, sealed his soul in exchange for his mission, and became a wandering spirit devoted to protecting children.
On the day of the party, Alex’s victory earns him popularity and an invitation to the celebration. At the party, Alex and the ninja confront Eppermint’s father, who is not the person the ninja seeks, as his name is misspelled as Eppermynt. Eppermynt provides Eppermint’s address. The ninja wants Alex to leave the party to pursue Eppermint, but Alex stays and, after the ninja insults Jessica using Alex’s voice, she kisses Odysseus, leaving Alex hurt and confused. They eventually reach Eppermint’s house, where the ninja’s attempt to kill Eppermint fails. It is then revealed that the ninja is Taiko Nakamura. When Alex refuses to help, Nakamura tries to murder Sean, after Alex had previously wished him dead, making Alex appear unstable and landing him in an asylum.
From there, the two strike a deal to bring Eppermint down without killing him. They visit Stewart, who provides the number of a drug dealer, delivering a kilogram of cocaine into the plan. Alex uses the cocaine to frame Eppermint for drug possession and follows Eppermint to the airport, where the two say goodbye. Eppermint travels to Thailand, and Nakamura springs onto him. Police find Eppermint carrying the cocaine‑stuffed doll, and Nakamura’s spirit leaves the ninja doll, leading to Eppermint’s arrest.
Back in Denmark, Alex learns that before leaving, Nakamura had sent Jessica a voicemail in his own voice, apologizing, and the two begin a romantic relationship. That night, Alex watches the news about street criminals and, inspired by Nakamura, embraces a vigilante path to protect Albertslund, vowing to keep his community safe.
