Dom Cobb (LEONARDO DiCAPRIO) is an extractor who heads, along with others on his team including Arthur (JOSEPH GORDON-LEVITT), into others' dreams in order to pull off corporate espionage of stealing secret and/or coveted information from his targets' subconscious. His latest client, Saito (KEN WATANABE), however, is more interested in doing the reverse with Robert Fischer (CILLIAN MURPHY). He's the heir to an energy conglomerate, and Saito would like to instill in him the notion of breaking up the company and thus its market monopoly.
Arthur, the team's point man, doesn't think it can be done, and that's due to needing to make the dreamer think they came up with the notion on their own while avoid the defensive projection characters the dreamer might conjure up as obstacles. Dom is enticed, however, by Saito promising to clear criminal charges against him that would allow Dom to return to his young kids, and thus he assures Saito the job can be done. He then sets out to assemble his team.
Along with Arthur, he adds college student Ariadne (ELLEN PAGE) to be the architect of building the necessary dream world; Eames (TOM HARDY) to serve as the forger who can assume the identity of others in dreams such as Robert's godfather, Browning (TOM BERENGER); and Yusuf (DILEEP RAO) as the chemist whose sedatives will insure the team can use dreams within dreams as part of their ploy without worrying that all of that will collapse all around them.
As the team conceives and then begins to execute their scheme -- where the temporal pacing speeds up in each subsequent dream level that's also affected by what occurs in the physical aspects of the next higher level and so on -- they must contend with various dangers and obstacles.
Among them is Fischer's natural mental defenses that conjure up armed men dead-set on killing the intruders, and the fact that if any of the team dies in any of the dreams, they'll end up forever trapped in a perpetual subconscious limbo. Then there's Dom's deceased wife, Mal (MARION COTILLARD), who shows up without notice, along with visions of their young kids, from his guilt-ridden mind, and tries to thwart the team's plans so that she can convince Dom to stay with her forever in the dream state.