With memories of his father's inexplicable death haunting him, Daichi follows a mysterious rainbow and travels to Tanegashima Island to investigate. Arriving on the island with mixed emotions, Daichi runs into a boy named Teppei who asks why Daichi has returned to this island. Daichi knows he’s returned there to fulfill a promise. Now, with panic alarms sounding and an unruly force known as the Killtgang quickly approaching to attack, Daichi must suit up for the invasion.