Igor, the dolphin soldier, is back. Auru, who formed a special bond with him, is undoubtedly the happiest to see him again. However, joy quickly turns to concern when Igor takes Zoom off with him. He wants to show him something: the sonars of the dolphins living around the island are disturbed and they end up beaching themselves. Even Zoom is affected by this strange sickness. Time is short so Yann and his friends need to discover the cause quickly.