Kevin is a human, from the planet earth.
Kevin Corrigan discovers the Cosmic Key in a crater, and starts pressing its buttons. Back at Castle Grayskull, Skeletor's second-in-command, Evil-Lyn, tracks the Key to Earth and prepares a small team of mercenaries to recover it.
Back in Whittier, Kevin and Julie are curious about the Key's origins, so Kevin, an aspiring musician, mistakes the object for a Japanese synthesizer, and takes it to a friend at a local music store to get a second opinion. While the pair is separated, Julie is attacked by Skeletor's forces, and saved by He-Man.
Kevin returns to the school, which has nearly been burned down as a result of the melee. The detective on scene, Detective Lubic, takes Kevin to Julie's house to look for her. Over the phone, Julie reveals to Kevin the importance of the Cosmic Key, but Lubic confiscates it, suspecting it to be stolen. Immediately afterwards, Evil-Lyn captures and interrogates Kevin, then leaves to acquire the Key from Lubic. Julie, along with He-Man and his comrades, meets up with Kevin and then proceeds to the music store, where Lubic has taken the Key for expert analysis. A battle ensues, during which Evil-Lyn, masquerading as Julie's dead mother, persuades her to steal the Key. Julie acquires it, and Evil-Lyn subsequently uses the Key's powers to open another doorway through which Skeletor arrives on Earth.
He-Man manages to retrieve the Key from Evil-Lyn, but Skeletor captures his friends, during the course of which Julie is fatally injured by Skeletor's magic and the second Key is damaged and Skeletor threatens to kill them all unless He-Man surrenders unconditionally. He-Man gives in and returns to Eternia as Skeletor's prisoner. Gwildor tries to repair the Key, but the Key's memory storage containing a special sequences of tones needed to return to Eternia was erased. Fortunately, Kevin remembers them, and uses a modern-day keyboard to recreate the tones and open a portal. Just as the opening of the portal is underway, Lubic blunders his way into the group and is transported along with them.
Skeletor continues to try to force He-Man to submit, but is interrupted by the arrival of Kevin and Lubic, who have just arrived through the portal that they'd managed to open. He-Man is freed and reclaims his sword, absorbing the power of Grayskull once more. Now evenly matched, He-Man along with his friends engages Skeletor's forces in combat, during which he breaks Skeletor's staff, causing him to lose his newfound powers. Skeletor continues to engage He-Man with his sword, but is ultimately vanquished and sent plummeting into a deep pit.
After He-Man's victory, Julie is healed by the Sorceress. Kevin and Julie bid farewell to their new friends before leaving through a doorway back to Earth. When Kevin finds Julie, she confirms their shared experiences were not just a dream and holds out a souvenir from Eternia: a blue, marble-sized sphere showing the image of He-Man in front of Castle Grayskull.