Following release from prison, Daniel "Danny" Ocean (George Clooney) violates his parole by traveling to California to meet his partner-in-crime and friend Rusty Ryan (Brad Pitt) to propose a caper. The two go to Las Vegas to pitch the plan to wealthy friend and former casino owner Reuben Tishkoff (Elliott Gould). The plan consists of simultaneously robbing the Bellagio, The Mirage, and the MGM Grand casinos. Reuben's familiarity with casino security makes him very reluctant to get involved, but when he starts to think of it as a good way to get back at his rival, Terry Benedict (Andy Garcia), who owns all three casinos, Reuben agrees to finance the operation. Because the casinos are required by the Nevada Gaming Commission to have enough cash on hand to cover all their patrons' bets, the three predict that, on the upcoming night of a highly anticipated boxing match, the Bellagio vault will contain more than $150,000,000.
Danny and Rusty recruit eight former colleagues and criminal specialists: Linus Caldwell (Matt Damon), a young and talented pickpocket; Frank Catton (Bernie Mac), a casino worker and con man; Virgil (Casey Affleck) and Turk Malloy (Scott Caan), a pair of gifted mechanics; Livingston Dell (Eddie Jemison), an electronics and surveillance expert; Basher Tarr (Don Cheadle), an explosives expert; Saul Bloom (Carl Reiner), an elderly con man; and "The Amazing" Yen (Shaobo Qin), an accomplished acrobat. Several of the team members carry out reconnaissance at the Bellagio to learn as much as possible about the security, the routines and behaviors of the casino staff, and the building itself. Others create a precise replica of the vault with which to practice maneuvering through its formidable security systems. During this planning phase, the team discovers that Danny's ex-wife, Tess (Julia Roberts), is Benedict's girlfriend. Rusty urges Danny to give up on the plan, believing Danny incapable of sound judgment while Tess is involved, but Danny refuses.
When the plan is put in motion, Danny goes to the Bellagio in order to be seen by Benedict, who, as expected, has him locked in a storeroom to be beaten by a bouncer called Bruiser. Bruiser, however, is a friend of Danny's, and he allows him to leave through a ventilation shaft, to meet with his team in the vault. Linus poses as a gaming commission agent and reveals to Benedict that one of his employees, Ramon Escalante, is actually Frank Catton, an ex-con. Linus and Frank stage a faux confrontation in Benedict's presence so that Linus can steal the vault access codes written on a piece of paper in Benedict's jacket. Yen is smuggled into the vault by the Malloy brothers to assist in triggering the explosive from the inside. Saul sneaks explosives into the casino vault by posing as a wealthy international arms dealer who needs especially secure safekeeping for his valuables and then pretends to have a heart attack that draws the security men's attention away from the vault monitors, and is subsequently treated by Rusty posing as a doctor.
Basher activates a stolen EMP device to temporarily disrupt the casino's electrical power, allowing Linus and Danny to drop down the elevator shaft undetected. As Benedict attempts to restore order following the power outage, Rusty anonymously calls him on a cell phone that Danny had earlier planted in Tess's coat. Rusty tells him that the vaults are being raided and that all the money will be destroyed if Benedict does not cooperate in loading half the money into a van waiting outside. Benedict observes video footage of the vault that confirms Rusty's claims and complies in moving the money but orders his men to follow the van after it departs and calls a SWAT team to secure the vault and the other half of the money. The SWAT team's arrival results in a shootout which causes the incineration of the half of the money left in the vault. After assuring Benedict that the casino is secure, the officers depart at Benedict's insistence.
Benedict's men following the van, discover it is being driven by remote control, and that, instead of money, it contains duffel bags full of flyers advertising prostitutes. Benedict realizes that the vault video feed he had been watching was pre-recorded, as the vault floor in the footage lacked the Bellagio logo, which had only recently been installed. A flashback reveals that Danny had used the vault replica to create the fake video Benedict had seen. The rest of the team posed as S.W.A.T. officers and took all of the money in the vault when responding to Benedict's call for police assistance. Benedict then returns to the room where he left Danny and finds him still there, apparently still being worked over by Bruiser, leaving him with no way to connect Danny to the theft. As Tess watches via security surveillance, Danny tricks Benedict into saying he would give up Tess in exchange for the money. Danny then says, "All right. I know a guy. We were in the joint together. Anybody pulls a job in the western US, he knows about it. Give me 72 hours. I'll find out who took your money". Benedict, humiliated, orders his men to escort Danny off the premises and inform the police that Danny is violating his parole by being in Las Vegas. Tess leaves Benedict and exits the hotel just in time to see Danny arrested. The rest of the team bask in the victory, silently going their separate ways one-by-one. When Danny is released after serving "three to six months" for his parole violation, he is met by Rusty and Tess, and the three drive off, closely followed by Benedict's bodyguards.

Question: Why did Danny have a replica of the casino vault replicated?
Answer: To practice how to get by it.
Alternative answer: To create a fake video