In the heart of a society where AIs are as common as ordinary people, Diva—nicknamed "Vivy" by her human friend—is the first autonomous humanoid AI in history. Like every other AI, Vivy is tasked with her own individual mission: to make everyone happy with her singing. She is programmed with a magnificent voice, but struggles to grasp the essence of a soul needed to touch a human's very core.

When Vivy seemingly suffers a technical issue, her personal archive is plagued by a strange cubical AI who calls himself Matsumoto. He claims to come from a hundred years in the future, where AIs have turned against mankind and caused a harrowing bloodshed.

With advanced capabilities, Matsumoto's specific duty is to, together with Vivy, alter the pivotal moments of history to prevent their peaceful coexistence from crumbling down. Ready to save lives, Vivy risks everything to aid Matsumoto's century-long mission, all while fighting an anti-AI terrorist organization and searching for the true meaning of happiness.