The film reveals that the "monster" was originally an ordinary flea on Charles, the laboratory's monkey, inadvertently enlarged by a botanical growth serum and vocal elixir. During the climactic confrontation atop the Eiffel Tower, Maynott fires at Francœur just as the potion's effects wear off, causing the creature to shrink back to microscopic size, making the public believe he was killed.
Maynott's murderous actions are captured on film by Émile and witnessed by Chief Inspector Pâté, leading to Maynott's arrest. Later at the cabaret, Lucille discovers Francœur alive and singing from within her hair. Using the recovered botanical formula, the group synthesizes a stable batch to restore Francœur to human size on demand, establishing his permanent place as a beloved cabaret star.