Saito is an immensely wealthy and powerful Japanese business magnate, and the CEO of Proclus Global.
When Cobb and Arthur's attempt to steal secrets from him fails, Saito reveals that he was testing them for a job and offers to hire them to perform inception on Robert Fischer, the son of Saito's biggest rival, to dissolve his father's business empire. Should Cobb succeed in doing so, Saito promises to clear Cobb's criminal status, which would allow him to finally return home to his children in the United States.
Although he has no specific expertise in extraction, Saito is still able to use his financial influence to join Cobb's team. Through his influence, he provides the opportunity for the team to isolate and sedate Fischer so they can enter his dreams long enough to plant the intended idea and even buys out an entire airline so that the team can accompany Fischer on a 10-hour flight to Los Angeles. To ensure the job succeeds, Saito insists on accompanying Cobb's team into the dream world.
During the first level of the dream, an imitation of New York, the team unexpectedly learn that Fischer's subconscious is militarized for his protection and Saito is shot when the team is attacked by a group of armed projections, forcing Cobb to reveal the true danger of the job. Although dying in a dream normally wakes the dreamer, the sedation required for the job means that death in the dream will drop the dreamer into Limbo, a dreamspace of raw subconscious and no limitations, resulting in the dreamer losing their sense of reality and time. With no way to heal Saito, the team is forced to complete the job as quickly as possible before he dies and falls into Limbo.
In the deeper levels of the dream, Saito's injury does not impede him as the time dilation in the lower levels mean the injury is taking longer to affect him. In the hotel dream, Saito is primarily an observer. He follows the team into the third level, a snowy mountain where he volunteers to lead Fischer to the hospital fortress where the inception will be finalized while Eames distracts the Fischer's subconscious projections and Cobb and Ariadne provide cover for Fischer. Saito's condition continues to worsen as his injury on the first level begins to affect him in the lower levels. After Fischer is shot by a projection of Cobb's wife, Mal, Ariadne suggests entering Limbo to find Fischer and send him back to the hospital dream. While she and Cobb enter Limbo, Eames sets the charges to detonate the fortress to wake them into the previous dream and has Saito defend Fischer. Saito uses a grenade to kill the projections who manage to enter the fortress, but dies before Fischer is revived and enters Limbo. Ariadne returns from Limbo and joins the rest of the team to awaken in the collapsing levels of the dream while Cobb remains in Limbo to find Saito, having anticipated that Saito has died in the time taken to find Fischer and overcome the projection of Mal.
Years later in Limbo, Cobb washes ashore and is brought into a large palace ruled by Saito, who has grown old in the dream world. Both struggle to remember their situation, but slowly regain memories of their lives in the real world. Finally, Cobb, noticing the constantly spinning totem, gives Saito the message that their world is not real. Cobb and Saito discuss going back to reality for a moment, and Saito reaches for Cobb's Beretta. The two of them later wake up from Limbo, presumably having shot themselves, and Saito immediately honors his agreement with Cobb by making a single phone call, clearing him of his criminal charges and allowing him to reunite with his children.