Saito is an immensely powerful and wealthy business magnate, and is the head of Proclus Global. When Cobb and Arthur's attempt to steal secrets from Saito fails, Saito soon reveals that he had been auditioning them for a specific job: an inception on Robert Fischer, the son of Saito's biggest rival, to dissolve his father's business empire. Should Cobb succeed at inception, Saito offers the means for Cobb to return to his children in the United States.
Though he has no specific expertise, Saito is still able to use his financial influence to join Cobb's extraction 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; Saito buys out an airline so the team can accompany Fischer on the 10-hour flight to Los Angeles. To ensure the inception is succeeds, Saito insists on accompanying Cobb's team into the dream.
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. Saito is shot in the process, forcing Cobb to reveal the true danger of the job: though dying in a dream normally wakes the dreamer, the sedation required for the job means that death in the dream will drop the dream into Limbo, a dreamspace of raw subconscious and no limitations, resulting in the dreamer losing his sense of reality and time. Saito's injury forces the team to complete the job as quickly as possible.
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 led 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.
Yeahs later in Limbo, Cobb washes ashore and is brought into a large palace, where an elderly Saito is found. Both struggle to remember their situation, but slowly regain memories of their former life. Finally, Cobb, noticing the constantly spinning totem, gives Saito a message: that the world is not real. Cobb and Saito discuss going back to reality for a moment, and Saito reaches for Cobb's Beretta. The film's subsequent events are left ambiguous to the audience. Both Cobb and Saito awake from Limbo, presumably having shot themselves, and Saito immediately honours his agreement to Cobb, allowing him to return to his family with a single phone call.