With principal photography underway on Oz The Great And Powerful, Sam Raimi's prequel to The Wizard Of Oz, Collider has revealed the official plot synopsis for the film:

Oz The Great And Powerful imagines the origins of L. Frank Baum’s beloved character, the Wizard of Oz. When Oscar Diggs (James Franco), a small-time circus magician with dubious ethics, is hurled away from dusty Kansas to the vibrant Land of Oz, he thinks he’s hit the jackpot—fame and fortune are his for the taking—that is until he meets three witches, Theodora (Mila Kunis), Evanora (Rachel Weisz), and Glinda (Michelle Williams), who are not convinced he is the great wizard everyone’s been expecting. Reluctantly drawn into the epic problems facing the Land of Oz and its inhabitants, Oscar must find out who is good and who is evil before it is too late. Putting his magical arts to use through illusion, ingenuity—and even a bit of wizardry—Oscar transforms himself not only into the great and powerful Wizard of Oz but into a better man as well."

Collider notes that Zach Braff also has a role in the film, as the Wizard's assistant, as well as providing the voice for his winged monkey sidekick. 

With Hollywood filming a prequel for every time honored film franchise, it was only a matter of time before a movie like Oz The Great And Powerful got produced. However, the difference between this prequel and others is that Oz has a wealth of talent behind it. We'll see if it can stand beside the 1939 classic when it's released on March 8, 2013. 

[via Collider]