It ends when The Guy says it ends

FBI agent Max Reed witnesses Dr. Tetsu Segawa - a researcher for the mysterious Chronos Corporation - being murdered. Dr. Segawa had stolen an alien device known as "The Guyver" from Chronos. College student Sean Barker, whose girlfriend's father was Dr. Segawa, finds the Guyver's hiding spot while watching the forensic team investigating the crime scene.

Sho and his friend Tetsurou stumble upon an odd alien artifact while walking through the woods. Then, the alien artifact breaks free of its metallic bonds and enters Sho's body, turning him into the Guyver. With this new power, Sho must do battle with the evil Chronos corporation and their genetically enhanced Zoanoids, who seek to get the Guyver back into their labs. No one close to Sho is safe from Chronos. He must fight.

Sean Barker became the unwilling host to an alien bio-armor known as the Guyver. A year ago he destroyed the Kronos Corporation, an organization of mutants who want the Guyver. Now he is trying to find why the Guyver unit forces him to fight and kill evil.