"Monster Strike: A Rhapsody Called Lucy -The Very First Song-" is a 2016 anime film that serves as a prequel to the "Monster Strike" anime series. The story centers on Lucy, a young girl with a mysterious past, who becomes entangled in a conflict involving powerful monsters and the fate of the world. Lucy possesses a unique ability to communicate with monsters, a skill that draws the attention of both allies and adversaries. Her journey begins when she encounters Ren Homura, a skilled monster striker, and his partner, Ayanokouji, who are on a mission to protect the world from a looming threat.

The narrative unfolds as Lucy, Ren, and Ayanokouji uncover the existence of a legendary monster known as the "First Song," a being of immense power capable of reshaping reality. The trio must navigate a series of challenges, including battles with rival monster strikers and the machinations of a shadowy organization seeking to harness the First Song's power for their own purposes. Along the way, Lucy's connection to the monsters deepens, revealing her pivotal role in the balance between humans and monsters.

The film explores themes of friendship, trust, and the coexistence of humans and monsters, set against a backdrop of high-stakes action and strategic monster battles. Key characters include Lucy, whose empathy and courage drive the story; Ren Homura, a determined and skilled striker; and Ayanokouji, whose loyalty and tactical expertise complement the group's efforts. The antagonist, a figure shrouded in mystery, seeks to exploit the First Song's power, creating a sense of urgency and tension throughout the narrative.

The plot builds toward a climactic confrontation, where Lucy and her allies must confront the true nature of the First Song and its connection to her own identity. The resolution hinges on the choices made by the characters, emphasizing the importance of understanding and cooperation between humans and monsters. The film serves as an origin story, setting the stage for the broader "Monster Strike" universe while providing a self-contained narrative focused on Lucy's journey and the origins of the First Song.