Paris, 1941. A family of scientists is on the brink of discovering a powerful longevity serum when all of a sudden a mysterious force abducts them, leaving their young daughter April behind. Ten years later, April (voiced by Academy Award nominee Marion Cotillard) lives alone with her dear cat, Darwin, and carries on her family's research in secret.

But she soon finds herself at the center of a shadowy and far-reaching conspiracy, and on the run from government agents, bicycle-powered dirigibles and cyborg rat spies. Undaunted, she continues her quest to find her parents and discover the truth behind their disappearance.