Tetsujin 28-gou

A scientist built giant robot warriors for wartime use, but his work was destroyed before deployment. Decades later, two surviving models are found and used for criminal purposes. A young boy, the scientist's son, investigates and learns that a more powerful robot is being sought. He locates it first, aided by a presumed-dead former colleague of his father. They stop the robot from being misused and commit it to peaceful purposes. With the help of a police officer and the scientist's colleague, the boy fights crime and later confronts alien invaders who bring their own giant machines to Earth.