Tobias the Hybrid Tram Engine's sense of identity is thrown into question when a mysterious enemy returns to the railway with a hidden agenda and throws everyone's lives into chaos.

Ever since Montague's defeat, Tobias the Hybrid Tram Engine has been feeling very confident about himself. More and more diesels are working on the island, and he even has a branch line of his own. However, not all engines strictly agree with Toby and his willingness to accept diesels, and Henrietta faces her own troubles of trust and acceptance. Then, a wrench is thrown into Toby's life when Oliver returns. Most of the engines abhor his presence on Sodor. Even Henrietta has doubts, but Toby takes it upon himself to give him a second chance. However, Oliver's dark past and his plans are slowly brought to light, and everything that defines Toby for who he is is thrown into question with the re-emergence of Oliver's oldest ally. Now, Sodor is falling apart at the seams, and Toby's very life is on the line. And it will take all of his strength to reclaim his soul from the Arcane Merchant.