After another day full of adventure, Yakari and his faithful mustang, Little Lightning, go to sleep. But when Yakari wakes up in the morning, Little Lightning isn't in the enclosure! Where did he go? Little does Yakari know that Little Lightning has to undergo many dangerous tests: ford a wild river, find a way out of a labyrinth or pass a trial by fire. Will he be able to handle them unscathed and return to Yakari?