Agnes, a 15-year-old Hulder girl from the Nebula realm, is tasked with a perilous mission to Earth. Her goal is to befriend a human boy named Steinar and earn his trust, so that when she reveals her cow's tail, he will believe she is a Hulder. Hulders are mythical, human-like creatures who rely on humans' belief in their existence for their life force. However, Agnes falls in love with Steinar and keeps her secret until a trio of Hulder hunters forces her to reveal her true identity. Shocked but believing her, Steinar watches as Agnes flees back to Nebula with the hunters in pursuit.
Upon her return to Nebula, Agnes is hailed as a hero for converting Steinar into a believer. Meanwhile, a betrayed Steinar on Earth vows revenge, feeling deceived by Agnes. Thinking her mission is complete, Agnes is stunned when Elder, the village leader, insists she return to Earth to retrieve a crucial pendant lost in the woods. Reluctant but compelled, Agnes understands the pendant's potential danger if it falls into human hands—it can open the portal to Nebula.
Back on Earth, Agnes finds the pendant in the bogs and decides to visit Steinar to apologise for her deception. Steinar rejects her apology, leaving her heartbroken. When Steinar's Uncle learns from Steinar about the Hulder hunters, he realises Agnes is in grave danger. Steinar and his Uncle rescue Agnes from the hunters in the woods, allowing her to escape back to Nebula with the pendant. In Nebula, with the help of Eversor, a cunning villager, Agnes uncovers that Elder has been sending Hulders on dangerous missions to Earth under false pretences. They expose Elder's deceit to the villagers, leading to her banishment. Agnes then returns to Earth to retrieve a second pendant stolen by the Hulder hunters.