Earth has regained peace, but a new enemy with space-time powers has arrived! Musashi, Cosmos' host, tells the UPG that the Ultra Warriors have been sealed inside an evil space-time castle by a warrior woman named Arina. The fate of the entire world will be at stake. Hikaru and Shou are not strong enough to face this enemy, but if they can learn to fuse their hearts, they stand a chance. But to do that, they first must survive the training of Ultraman Zero!