When Astro is activated the city's power starts to drain, and he must try and stop the robot Magnamite from destroying everything. Armed with Kokoro (the ability of free will) Astro discovers his purpose for being activated.

When a friendly robot-dog invites Astro to watch a game of Rocket Ball, he looks forward to a fun day at the ballpark, Instead, the game turns chaotic when the robot players start destroying the stadium and threatening the human spectators. Astro must find a way to save the humans and get the robot athletes under control!

Dr. Tenma builds a powerful robot named Atlas whose abilities rival Astro's, But Atlas has a rebellious nature and begins to destroy Metro City. Astro must save the city from Atlas, while beginning to learn the secrets of his own past.