"Happy Heroes League: Adventure in the Mechanical Fort" is the fifteenth installment of the "Happy Heroes League" series. The story likely takes place on Green Star, known as the "Cosmic Garden." While on a mission, the five heroes accidentally discover a hidden program code within themselves that can control them. During the investigation, the heroes find that all clues point to the Wujin Company of the Cosmic Development Group. In order to unravel the mystery, the heroes embark on a journey to uncover the truth, and the conspiracy of Wujin Company gradually comes to light. It turns out that Wujin Company once possessed the super-powered mechanical stones that gave birth to the heroes and attempted to transform them into their own weapons to dominate the universe. After learning the truth, the heroes join forces to destroy Wujin Company and resolve the cosmic crisis.