"Eiga Tamagotchi: Himitsu no Otodake Daisakusen!" is a feature-length anime film based on the popular Tamagotchi virtual pet franchise. The story centers on Mametchi, a curious and intelligent Tamagotchi character, and his friends as they embark on a mission to uncover the secrets of a mysterious musical instrument called the Otodake. The Otodake is said to have the power to bring happiness to anyone who hears its music, but it has been lost for years. When Mametchi and his friends discover a clue about its whereabouts, they set out on an adventure to find it.

The narrative unfolds in Tamagotchi Town, a vibrant and colorful world inhabited by various Tamagotchi characters. Alongside Mametchi, the main cast includes Memetchi, a fashion-loving Tamagotchi; Kuchipatchi, a cheerful and food-obsessed character; and Lovelitchi, a kind-hearted and empathetic Tamagotchi. Together, they face challenges and solve puzzles to locate the Otodake, encountering both allies and obstacles along the way. Their journey takes them through diverse environments, from bustling towns to serene natural landscapes, each with its own unique charm and challenges.

As the story progresses, the group learns about the history of the Otodake and its connection to the happiness of Tamagotchi Town. They discover that the instrument was hidden to protect it from those who might misuse its power. The characters must work together, using their individual strengths and teamwork, to overcome the trials that stand between them and the Otodake. The film emphasizes themes of friendship, perseverance, and the importance of spreading joy.

The climax of the story involves a confrontation with a group of antagonists who seek to use the Otodake for their own selfish purposes. Mametchi and his friends must outsmart these adversaries and ensure that the Otodake is used to bring happiness to everyone in Tamagotchi Town. The resolution highlights the power of unity and the positive impact of selfless actions, reinforcing the core values of the Tamagotchi franchise. The film concludes with a celebration of their success, leaving viewers with a sense of warmth and optimism.