Movie quote "...or so they thought"?

This is a movie that I believe is a comedy. If not it's only semi-serious. There is a narrator who has a slight british accent and he talks about the main characters who were headed further along in their adventure, and he ends the scene with the line "or so they thought." And he says it very deliberately and seriously. He may have been reading the story from a book If I had to guess I'd say it was made in the late 90's, early 2000's but that may be way off. I can't think of what movie this is and it's driving me nuts. I know its not a lot to go on but I feel like someone will know what I'm talking about.