The film follows Dr. Parnassus, his daughter, and his assistant as they travel around trying to let people into their imagination, to see all that is possible. When a deal with the devil comes due, the immortal Doctor Parnassus (Christopher Plummer) must renegotiate the pact to save his daughter. Now, with the help of his mystical theater troupe and a mysterious stranger (Heath Ledger/Colin Ferrel/Johnny Depp/Jude Law), Parnassus attempts to right the wrongs of his past.
Another interesting thing to note is that Gilliam was very selective on who would step into Ledger's shoes. He wanted to keep it to Ledger's personal friends, so it would serve as an homage of sorts. The three men also gave their payment from this role to Matilda, Heath Ledger and Michelle Williams' daughter, so her financial future would be secure.
All these lead to a different viewing of the film than if it were viewed in a vacuum, if you had no knowledge of the Heath Ledger tragedy.
Also of note, Andrew Garfield plays the eager young assistant. This was the first I'd seen him, but he's since blown up with roles in The Social Network, Never Let Me Go, and will be the new Spider-Man. I enjoyed him in this role, but never thought he'd become so well known so quickly!
The film itself is a unique viewing experience. With the dreamscapes in the Imaginarium, I'm simultaneously in love with it while wishing it had a bigger budget to make them even more fantastical. There were no limits on what they could look like, no limits on what they could do in there. If you can, try to stream this to your television through a game console or an HDMI cable hookup. It definitely needs more than a small laptop screen to be fully enjoyed.