When trouble strikes at the Blue Mountain Quarry, Thomas is sent to help his Narrow Gauge engine friends. While shunting and hauling, he catches a glimpse of a small engine trying to hide, yet no one will provide any clues to the engine's identity. Thomas is determined to get to the bottom of the mystery, but discovering the truth is harder than he imagined! Will Thomas be able to save his new friend? Or will the mysterious engine be sent away from Sodor forever?