Arctic-based Gaira aliens plot Earth's conquest. Dr. Yan, their captive, builds Groizer X, a giant transformable robot. His daughter Rita escapes with Groizer X to Japan, meeting pilot Joe Kaisaka. Together, they battle the alien invasion.

The Gaira aliens, hidden in the Arctic, plan to conquer Earth. Captured and forced to work for the aliens, Dr. Yan creates the ultimate weapon, a transformable aerial robot called Groizer X. Groizer X was 100 meters tall and weighed 1200 tons. Entrusted to his daughter Rita, Groizer X escapes the clutches of the Gaira and lands in Japan, where pilot Joe Kaisaka meets a wounded Rita. Taking up the controls of Groizer X, Jo and Rita fight against the Gaira invasion.—Moh-X