An elderly couple is offered to meet with their dead son by strange people in exchange for herbal medicine.

Fifteen years ago, three childhood friends, Geki, Sherry and Okuma, were star-gazing and became inspired to go into space. As adults, Geki and Okuma join a mission to Mars, but their space shuttle goes missing before they could reach their destination. A year later, a monster attacks and a mysterious silver-colored warrior appears.

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!