Allegro, an older dog, initially serves military handlers alongside two younger watchdogs. Abandoned by enemy soldiers due to his age and perceived weakness, he allies with Coda the cat and Largo the donkey. Allegro possesses the unique ability to understand and translate between cat and donkey languages, a skill essential for the group's unity as they search for Trio, Coda's missing human companion.

Allegro participates in forming a musical group with the other animals, leveraging their shared interest in music as a strategy to locate Trio. The group encounters Rondo, an extraterrestrial being who provides transformation leaves. Using a leaf, Allegro gains the ability to assume human form, allowing him to spread Rondo's message of peace alongside the transformed Bremen 4 ensemble.

As a human, Allegro contributes musically within the group, particularly through trumpet playing. The Bremen 4 achieve popularity as performers in Bremen, though their initial focus later shifts toward resistance against the militaristic regime led by Presto. During a guerrilla assault on Presto's castle, Allegro guides fighters through the dungeon's sewer system. When he struggles to retrace the escape route, he reverts to his canine form to navigate by scent, ensuring the group's successful infiltration.

Following the conflict, Allegro and the other animals abandon their human forms permanently. They dedicate themselves to disseminating Rondo's anti-war philosophy among Earth's animal populations, engaging with other creatures such as Unico and the lion cub Leo to promote global unity and peace.

Allegro's military background occasionally resurfaces tactically, like coordinating the sewer route during the siege. His loyalty transitions from human masters to his animal companions, and his role evolves from translator and musician to an active participant in strategic resistance.