"Gekijōban Rokumon Tengai Mon Colle Knight: Densetsu no Fire Dragon" is a theatrical anime film based on the "Mon Colle Knights" franchise, which revolves around the adventures of two young protagonists, Rokuna and Mondo, who are tasked with protecting the balance between the human world and the Monster Realm. The story begins when a legendary Fire Dragon, a powerful and ancient creature, is awakened from its slumber, threatening both worlds with its destructive power. Rokuna and Mondo, armed with their Monster Collection devices, must embark on a perilous journey to stop the Fire Dragon and uncover the truth behind its sudden resurgence.

The narrative explores themes of courage, friendship, and the consequences of tampering with ancient forces. Rokuna, a spirited and determined girl, and Mondo, her resourceful and brave companion, work together to gather allies from the Monster Realm, including various magical creatures, to aid them in their quest. Along the way, they face numerous challenges, including rival collectors and the Fire Dragon's overwhelming power. The film delves into the mythology of the Monster Realm, revealing the Fire Dragon's connection to an ancient prophecy and its role in the balance of the two worlds.

As the story progresses, Rokuna and Mondo uncover secrets about the Fire Dragon's origins and the true intentions of those who seek to control it. The climax centers on a high-stakes battle where the protagonists must use their wits, teamwork, and the bonds they've formed with their monster allies to confront the Fire Dragon and prevent catastrophe. The resolution ties together the themes of the narrative, emphasizing the importance of understanding and respecting the natural order, while leaving room for future adventures in the "Mon Colle Knights" universe.