A mysterious girl named Lemon Sunomono suddenly appears in front of Shinnosuke Nohara, claiming to be an emissary of his hero, Action Kamen. She brings a message that Shinnosuke is to become an action spy, and he eagerly agrees to begin training as her partner. Lemon, a seven-year-old spy who possesses remarkable physical abilities and knowledge, has been trained since childhood by her parents in the Republic of Sukashipestan and works under the command of the nation's two queens. Believing he is fighting for justice under Action Kamen's direct instruction, Shinnosuke completes his spy training and receives his first mission: infiltrate the laboratory of the evil professor in the Hedelna Kingdom and recapture a stolen capsule known as the Action Power Capsule.

The mission proceeds as Shinnosuke and Lemon break into the laboratory, but the situation takes an unexpected turn when the professor warns them that they have walked into a trap. Shinnosuke refuses to believe that Action Kamen would deceive him and takes the capsule anyway. However, Lemon begins to uncover the real purpose behind the mission and the true intentions of the queens she serves. The two queens, Narao and Yosuru, are the main antagonists whose hidden agenda involves making the world suffer through the smell of flatulence, stemming from their own childhood humiliation caused by it. The experimental sweet potato jelly Megahegaderu II lies at the center of their scheme. After learning the truth, Lemon breaks free from the conditioning of absolute obedience instilled by her parents and joins forces with Shinnosuke and his family to stop the queens' plan. The two children ultimately turn the weapon against the queens by force-feeding them the Megahegaderu II, causing a massive explosion of flatulence that destroys the laboratory and sends the villains flying away. Following these events, Lemon and her parents go into exile in the Hedelna Kingdom, where her parents vow to become better parents after witnessing their daughter risk her life.