Spirou and Fantasio are spending a few days at Champignac Castle, keeping an eye on the estate while the Count is away visiting family. One night, as they are preparing to watch a comet pass in the sky, Fantasio's clumsiness causes them to lose consciousness. They awaken to the very noisy arrival of de Champignac aboard one of his strange machines – except that the old man claims to be not the Count himself, but one of his descendants ... back from the future!