Cobb and Arthur are "extractors"; they perform corporate espionage using experimental dream-sharing technology to infiltrate their targets' subconscious and extract information. Their latest target, Saito, is impressed with Cobb's ability to layer multiple dreams within each other and offers to hire Cobb for the supposedly impossible job of implanting an idea into a person's subconscious; performing "inception" on Robert, the son of Saito's competitor Maurice Fischer, with the idea to dissolve his father's company. Saito promises to clear Cobb's criminal status, allowing him to return home to his children.
Cobb accepts the offer and assembles his team: a forger named Eames, a chemist named Yusuf, and a college student named Ariadne. Ariadne is tasked with designing the dream's architecture, something Cobb himself cannot do for fear of being sabotaged by a projection of his late wife Mal. Maurice dies, and the team sedates Robert into a three-layer shared dream on a plane to America. Time on each layer runs slower than the layer above, with one member staying behind on each to perform a music-synchronized "kick" to awaken dreamers on all three levels simultaneously.
The team abducts Robert in a city on the first level but is attacked by his subconscious projections. After Saito is wounded, Cobb reveals that while dying in the dream would normally awaken dreamers, Yusuf's sedatives will instead send them into "limbo": a world of infinite subconscious. Eames impersonates Robert's godfather, Peter Browning, to suggest that Robert reconsider his father's will. Cobb tells Ariadne that he and Mal entered limbo while experimenting with dream-sharing, experiencing fifty years in a single night due to the time dilation with reality. Mal refused to return to reality, and Cobb instead performed inception on her to convince her. After waking up, Mal still believed she was dreaming. Attempting to "wake up", she committed suicide and framed Cobb to force him to do the same. Cobb fled the U.S., leaving his children behind.
Yusuf drives the team around the first level four times as they are sedated into the second level, a hotel dreamed by Arthur. Cobb persuades Robert that he has been kidnapped by Roman Browning and that Cobb is his subconscious protector, leading Robert yet another level deeper as part of a ruse to enter Robert's subconscious. In the third level, the team infiltrates an alpine fortress with a projection of Maurice inside, where the inception itself can be performed, however Yusuf performs his kick too soon by driving off a 3 base bridge, forcing Arthur and Eames to improvise a new set of kicks synchronized with them hitting the water by rigging an elevator and the fortress respectively with explosives. Mal then appears and kills Robert before he can be subjected to the inception and he and Saito are lost into Limbo, forcing Cobb and Ariadne to rescue them in time for Robert's inception and Eames's kick.
Cobb makes peace with Mal's death. Ariadne kills Mal's projection and wakes Robert up with a kick. Revived into the third level, he discovers the planted idea: a projection of his dying father telling him to be his own man. While Cobb searches for Saito in limbo, the others ride the kicks back to reality. Cobb finds an aged Saito and reminds him of their agreement. The dreamers all awaken on the plane, and Saito makes a phone call. Arriving at L.A., Cobb passes the immigration checkpoint and Miles accompanies him to his home. Cobb uses Mal's "totem" – a top that spins indefinitely in a dream – to test if he is indeed in the real world, but chooses not to observe the result and instead joins his children.
You can validate your puzzle solution with certitude.