Professor Goldthwait Higginson Dorr (TOM HANKS) is no instructor, but rather a conman and criminal who's determined to rob the floating Bandit Queen casino boat. The only problem is that its daily take is stored in a highly secure vault in the counting house located onshore.
Rather than go through the front door, however, the professor has decided to tunnel his way to riches and thus rents a room from Marva Munson (IRMA P. HALL). She's an older and sweet churchgoing lady who's complained so many times to Sheriff Wyner (GEORGE WALLACE) about so many trivial matters that he no longer pays her any heed.
Dorr easily convinces Marva that her cellar's earthen walls are acoustically perfect for his band to practice their gospel-inspired, baroque music. Among the players is Gawain MacSam (MARLON WAYANS), a casino custodian who will be their inside man; the stoic General (TZI MA) who has experience with tunnels; munitions expert Garth Pancake (J.K. SIMMONS) who brings in an outsider, Mountain Girl (DIANE DELANO), for help; and Lump (RYAN HURST), a somewhat dimwitted football player who will serve as their muscle.
Using a host of lies and distractions to keep Marva in the dark about their scheme, the team sets out to tunnel to the vault, break-in, and make off with the loot. When Marva stumbles onto their plot, however, the group must act before she turns them in to Sheriff Wyner. From that point on, they decide they must silence her lest their plan go awry, all of which leads to a series of unexpected, black comedy repercussions.