Mr. Conductor's supply of magic gold dust, which allows him to travel between Shining Time and Thomas's island, is critically low. Unfortunately, he doesn't know how to get more. Meanwhile, Thomas is fending off attacks by the nasty diesel engines. Getting more gold dust will require help from Mr. C's slacker cousin, his new friend Lily and her morose grandfather, plus the secret engine.
Reviews
It's entirely possible that sending the audience out feeling lousy was intentional
This is one of the few movies I've ever seen where the whole audience broke into spontaneous, loud applause a third of the way in.
Each character in this movie — down to the smallest one — is an individual rather than a type, prone to spontaneous changes of mood and sometimes amusing outbursts of pettiness or ill humor.
One of the film's great tricks is that, for a time, you think it will go down a rabbit hole of unrealistic glorification.