Eels have puzzled naturalists since Aristotle, who guessed they grew from mud, and this film traces the centuries-long hunt for where they actually come from. No one has ever observed eels mating or found their eggs in the wild, and the film follows the biologists and fishermen still trying to solve that gap. It moves from European rivers and the Sargasso Sea, where Danish researcher Johannes Schmidt first tracked larval eels back toward a suspected spawning ground, to the freshwater eel populations that vanish each autumn on a migration no camera has fully documented. Interviews with marine scientists lay out competing theories about depth, temperature, and the strange metamorphosis eels undergo before their final journey, while archival material shows how the mystery shaped folklore as much as science. The film treats the eel's life cycle as an open case rather than settled fact, and it is genuinely unresolved: even now, the species' full reproductive story has never been directly witnessed.