Twelve years ago, two giant robots fought each other in the Kingdom of Rimgarde, causing massive destruction. In the present, two sisters, Rena and Yui, enjoy breakfast. Seeing Rena's annoyance at her taking on more responsibility despite being the younger sister, Yui invites to go out after school to cheer her up. However, as Rena waits for Yui to arrive from school, she is confronted by her old acquaintance Kei, whom tells her to come with her if she wants to learn what really happened twelve years ago...