Professor Otoi has received an invitation to a robotics convention being held in the floating city of Ryukeion. It's being hosted by the research organization SINKTANK. Nobuhiko's father, Masanobu, offers to take the Human Form Robots Signal, Pulse, and Elara along with Nobuhiko and Chris, but he appears to have his own purposes for making the trip. After they arrive in Ryukeion City, Nobuhiko meets a mysterious girl, and Chris unexpectedly reunites with her sister. But unknown to them all, a diabolical scheme is about to unfold around them.
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