Thomas & Friends: The Adventure Begins is a 2015 British animated adventure comedy film produced by HIT Entertainment and animated by Arc Productions. It serves as a prequel to the British television series, Thomas & Friends. It is based on the first two books of The Railway Series: The Three Railway Engines and Thomas the Tank Engine, which were previously adapted into the first seven episodes of the show's first season in 1984. It commemorates the 70th anniversary of The Railway Series, making it the first installment to officially adapt a Railway Series story since Series 4.

John Hasler and Joseph May take over the voice of Thomas the Tank Engine as new voice actors in the UK and the US, respectively. Rob Rackstraw joins the US cast as James the Red Engine (he would join the UK cast later the same year), Keith Wickham takes over the role of Sir Topham Hatt in the US, and Christopher Ragland takes the role of the Troublesome Trucks in the UK and US, respectively. It also co-stars the voices of Tim Whitnall, Teresa Gallagher, William Hope and Kerry Shale, voice actors who were already working on the series at the time of the special's production. The production of the special itself was fast-tracked into Arc Productions' schedule for the franchise's 70th anniversary, and as a result, the 21st series of the show had eight episodes cut from its run when the Big World! Big Adventures! rebrand began development.