Based on the previews I saw for The Departed, I pretty much had no idea what I was getting myself into when I went to see it. I didn't think I would see much more than a gun-filled action film with some famous actors thrown into the mix.

Luckily, I was pleasantly surprised.

The Departed, directed by Martin Scorsese, is essentially about rats.

No, I'm not talking about the little furry critters -- I'm talking about people who work for organizations and spend their time infiltrating other organizations. Leonardo DiCaprio plays a Boston policeman named Billy Costigan who goes undercover and becomes part of an organized crime network. He tries to help the police arrest and convict Frank Costello, the big boss of crime, played by Jack Nicholson.

While this story line might be enough to make an interesting movie, it gets more complicated.

Frank Costello has sent his own undercover man into the police force. Colin Sullivan, played by Matt Damon, works his way through the Boston police department and eventually becomes a lead detective. Costello wants Sullivan to find out who the undercover agent in the crime network is, but then the police department catches wind of something and tells Sullivan to root out the rat working among them. That is, of course, Sullivan himself.

The movie stretches this idea out for 2 1/2 hours. It requires that much time to untangle the mess made by the plot, and even the ending felt rushed as loose ends were resolved.

I don't want to give away what happens, but I will say that it felt like the ending of a Shakespearean tragedy -- there was one death after another.

On that same note, when the movie was over, I still had questions about what just happened. By the time the credits started rolling, I wondered where certain characters' loyalties lay and who was actually on the good side or the bad side.

Although this is annoying with many movies, I actually liked the somewhat ambiguous conclusion of The Departed.

The way it ended kept me thinking about possibilities and other elements of the story long after I walked out of the theater.

One theme that was stressed in The Departed is that you never know whom to trust.

Both of the main characters were not who they appeared to be, and that made the movie difficult to follow at times.

The actors' performances made the movie especially believable. Matt Damon played a perfect hot-shot police detective, and Leonardo DiCaprio had a scruffy demeanor that gave him an aura of delinquency.

Plus, you can't go wrong with Jack Nicholson playing the bad guy, especially since The Shining came out. Sometimes it's hard to picture actors playing anything besides their major roles, but in this movie it was easy.

The Departed kept my attention from beginning to end. Normally I'm not a big fan of action films, but the believable characters, complex plot and overall great story turned this movie into something I wouldn't mind seeing again.

Grade: A-