Let me just start off by saying that this is not a movie for the squeamish. People die horrible deaths in this movie.
With that out of the way....The movie is, of course, about the 1972 Olympic massacre of Israeli athletes committed by a splinter faction of the PLO. The Israelis, with Golda Meir as PM, launch a large revenge operation that will target the planners of the massacre. For the job, the Mossad recruits Avner (Eric Bana) and 4 others for his assasination team.
As you would expect, they are initially successful. But, as their operation drags on, and as they begin to realize that the people they are killing are being replaced alomst right away, they begin to question the usefulness of their mission.
Avner especially struggles with the inner turmoil between following his orders and the realization that his mission isn't changing anything. One way Spielberg does this is through a series of vignettes to the massacre in '72. This happens whenever Avner begins to waver in justifying his mission.
However, if one has any knowledge of the events, one quickly sees how much artistic licence Speilberg has taken with the actual events. Yet, all things considered, this is an excellent movie. It starts to drag on a bit at the end, but I highly recommend it nonetheless.