What exactly is Sweeney Todd about?

i dont really get the preview ive seen enough to figure anything out about the plot.

He was a guy that was wrongfully put in jail, no one remember him so when he gets out he becomes a barber and starts to kill the ppl that put him in jail.

http://www.imdb.com/title/tt0408236/

Basically, Sweeney (depp's character) is wanting revenge from judge turpin (alan rickman character) who falsely accused sweeney so that when sweeney was in jail, judge turpin had raped his wife. He adopted Sweeney's daughter and intends to marry her.

When Sweeney gets out of prison years later, Sweeney meets up with his former landlady and a corrupt baker Mrs. Nellie Lovett (Helena Bonham Carter) who fuels his desire for revenge into a murder frenzy (Sweeney slices the throat of his customers) . Mrs. Lovett's meat pie business flourishes after the flesh of Todd's victims is added to her recipe to get rid of the bodies.

*very* bloody and gory...lots of songs in it since it is a musical ^^

Sweeney Todd a.k.a Benjamin Barker returns to London after being sent away by Judge Turpin with the help of a sailor, Anthony Hope. He opens a barber shop above Mrs. Lovett's Meat Pie Shop were she sells "the worst pies in London." With the help of Mrs. Lovett, Todd tries to rid of all the people who have ever done him wrong and hopes to be reunited with his daughter, Joanna, who is now Judge Turpin's ward.

i think its based on a play

Basically... there was a barber (Benjamin Barker) and his wife, and they were a happy young couple, but when a corrupt judge wanted the barbers wife for himself, he sent the barber away for life on a false charge. The barber escaped after 15 years in prison, and he came back an angry, twisted, vengeful madman. He names himself Sweeney Todd and sets up a barber shop above the bakery of the deceiptful, cunning Mrs. Lovett. Eventually, Todd slays a man and Mrs. Lovett devises a plan to get rid of the body - put it in her pies!

That's pretty much the first half of the movie. You'll need to see it to find out more.