DiCaprio, Joseph Gordon Levitt and Ellen Page play people who can create and enter someone's dreams and steal ideas. The idea being that the thoughts in someone's head are a lot less well guarded than those ideas found in a safe. Consequently, stealing someone's thoughts is a lot easier than stealing paper.
The team is approached to do the opposite in this film, however. Instead of taking an idea out, they are asked to insert a thought into Cillian Murphy's head. The hope being that once it's been inserted it'll grow into the fully formed action they want.
In theory, this isn't a bad film. It's enjoyable enough, but it's muddled in places. The team have to create dreams within dreams for the insertion to work, and I found the jumps between the dreams somewhat messily handled. Others might disagree, but I stand by this point because I don't think the film flowed as well as it could have done.