Cobb reveals to Ariadne that he knows inception is possible because he did it to his wife. He implanted the idea that the world she is living in is not real, allowing them both to escape the deepest subconscious. The idea grew, causing her to kill herself in real life. Cobb talks to Saito, now an old man, in the deepest layer of subconscious and reminds him about their arrangement. Saito reaches for his gun. Cobb wakes up, passes through customs and meets his father. He goes home and sees his kids faces. The last shot is of Cobbs totem, the spinning top... either spinning forever which means hes in a dream or about to topple which means hes in reality...