Inception Explained

Cobb and Ariadne find Cobb's projection of his dead wife Mal who has Fischer tied up.

Cobb explains that he'd previously performed inception on Mal. They experimented with dreams within dreams and ended up in limbo. At that depth the mind is so far removed from reality that for decades in limbo they both believed limbo was the real world.

Cobb finally realised it wasn't and planted that idea in Mal's mind. They killed themselves in Limbo, and awoke in the real world. But the idea had been planted and Mal was convinced that reality was a dream too, so she killed herself again, this time for real.

Cobb refuses to stay in limbo with his projection of Mal, so she stabs him.

Ariadne shoots Mal dead. She's only a projection, so stays dead.

Cobb does not appear seriously injured and tells Ariadne he will find Saito in limbo.

Ariadne pushes Fischer off the balcony, his death in limbo combined with a defibrillator reviving his body in Level three awakens him in Level three.

Ariadne feels the explosion in limbo and jumps off the balcony herself. The combination of the jump in limbo and the exploding fort awaken her in Level three.

Back in limbo Cobb finds Saito. The minutes in Level three between Saito and Cobb entering Limbo have felt like decades in limbo and Saito is an elderly man. Cobb reminds him that they're in limbo and Saito picks up a gun, presumably to kill them both.