Manabu Yuki and his older brother Mamoru are boastful about their father Wataru, the active head of the Space Defence Force (SDF) platoon on Sirius. One day, Sirius SDF Special Train 001 (Big One) with Wataru onboard drops in on planet Tabito, and father and son can meet again. However this is but a short break as Big One is ordered to scramble to rescue Train 707. Manabu and Mamoru reluctant to part from their father secretly get on Big One and set out on a trip to the dangers of the universe. Then, after rescuing the train, Big One is suddenly attacked by a squadron of spacecraft...