"Soreike! Anpanman: Rhythm de Utaō! Anpanman Natsu Matsuri" is a musical anime special centered around the beloved character Anpanman and his friends as they prepare for and participate in a lively summer festival. The narrative unfolds in the whimsical world of Anpanman, where the characters are anthropomorphic food items with unique abilities. Anpanman, the titular hero, is a bread-headed superhero who helps those in need by sharing pieces of his head, which is made of anpan (sweet bean-filled bread).

The story begins with Anpanman and his friends, including Shokupanman, Currypanman, and Melonpanna, eagerly preparing for the annual summer festival. The festival is a joyous occasion filled with music, dancing, and games, and the characters are excited to showcase their talents. However, the festivities are threatened when Baikinman, a mischievous and self-serving antagonist, devises a plan to disrupt the event. Baikinman, along with his sidekick Dokin-chan, aims to steal the spotlight and create chaos, driven by his envy of Anpanman's popularity.

As the festival progresses, Anpanman and his friends face various challenges posed by Baikinman, including sabotaged performances and stolen festival items. Despite these obstacles, the group works together to overcome the disruptions, emphasizing themes of teamwork, perseverance, and kindness. The narrative is interspersed with musical numbers, where characters sing and dance to upbeat tunes, adding a vibrant and engaging layer to the story. These musical sequences are integral to the plot, as they highlight the characters' determination to keep the festival spirit alive.

The climax of the story occurs during the grand finale of the festival, where Anpanman and Baikinman engage in a musical showdown. Through their performances, Anpanman demonstrates the power of positivity and selflessness, ultimately inspiring Baikinman to reconsider his actions. The resolution sees the festival successfully concluded, with all characters coming together to celebrate the joy of community and friendship. The special ends on a heartwarming note, reinforcing the core values of the Anpanman series: compassion, resilience, and the importance of helping others.