The series featured many firsts for the franchise, including the start of the story in a movie rather than the series proper. Ultraman Cosmos: The First Contact was the prequel to the series and took place ten years before episode 1. In it, 9-year-old Musashi Haruno encoutners the being of light, Ultraman Cosmos (ウルトラマンコスモス, Urutoraman Kosumosu?), and befriends him as the two confront a threat to Earth.

Ten years later, Musashi, now 19 and a member of Team EYES, once again encounters the being of light from his childhood. The dark being Chaos Header has appeared and is corrupting the monsters of Earth, making them become ravenous and violent creatures that threaten humanity. Through Musashi's will to see the beasts and humans live in peace, Cosmos grants him a new power that will allow him to heal the corrupted creatures.