The head bank robber, Dalton (Clive Owen), forces all the hostages to wear the same clothing as the robbers making the robbers and hostages all look alike.
Dalton only robs the safe deposit box of the bank's founder which contains diamonds and evidence of the founder's (Christopher Plummer) relationship with the Nazis which is how the founder gained all his wealth.
At one point during the hostage negotiations Dalton speaks with the head negotiator, Keith Frazier (Denzel Washington) and Dalton tells how he will simply walk out of the bank. Frazier also makes small talk explaining to Dalton about how he can't afford an engagement ring for his girlfriend.
The hostages, including all the robbers except Dalton are set free. Remember, they are all dressed alike. Dalton could be identified by Frazier so he doesn't leave. The police find that nothing has been stolen as there is no record of the safe deposit box, all the money is still in the vault and there is no evidence of any of the robbers.
Frazier discovers that one of the safety deposit boxes was not in the records and opens it. Inside is a large diamond ring with a note that says to "Follow the Ring" which ultimately will implicate the bank founder to his Nazi ties.
During the robbery Dalton had built a false wall in the storage room and stays inside it for four days after the robbery. As Dalton is finally leaving the bank he bumps into an unaware Frazier who is entering the bank.
Later, Frazier finds a diamond (for his girlfriends ring) that Dalton placed in his pocket during the bump and he figures it all out.
(Links open in new window)
Read more about this film at The Internet Movie Database.