Well, I picked an easy one here. How to talk about this in a way that makes sense but doesn't involve just listing the plot...
Um.
The film is set at the traveling show which bears the name of the film. It centres on getting people to pay to step through a mirror which will take them to the world of their dreams. This turns out to be exactly what it does, sending people into a world generated by their own imagination. As you may expect, this is not always a place filled with sunshine and fun.
Christopher Plummer plays Dr. Pernassus, a wizened figure, who is assisted in his show by Anton (Andrew Garfield), Percy (Verne Troyer) and his daughter Valentina (Lily Cole). The Doctor soon runs into an associate of his, Mr. Nick (Tom Waits) and they also take on a new hand, Tony (Heath Ledger)
The film is stylistically exactly what you would expect from Gilliam, with slightly quirky real world scenes (given the carnival nature of the show) and impressively realised dreamscapes within the imaginarium. There is a vein of humour running throughout.
Whether you'll like this or not depends to some extent on how much you like Gilliam's films. I do, and I enjoyed this, but I did find the pacing to be verging on too slow at times and there is a fair deal going on at the same time.
My rating, as always, reflects what I thought and so should be tempered if this is not really your cup of tea.