The film The Departed follows the stories of three particular men, each of whom lead vastly different lives in a corrupt, current-era boston. The first man we’re introduced to is Frank Costello (Jack Nicholson), a ruthlessly efficient mobster who has amassed a close following of loyal associates. The next man is Colin Sullivan (Matt Damon) whom we’re introduced to when is only a boy being raised by his grandmother, when Costello is giving him a hefty supply of groceries, courtesy of the small deli he is extorting. After a few more interactions between Costello and Sullivan, it is clear that Costello is a bit of a father figure and mentor for Sullivan, whom we next see as a graduated police cadet. Sullivan gradually works his way up the ranks to work in the SIU (Special Investigations Unit) of the Massachusetts state police; a unit responsible for taking down mobsters much like Costello. And finally we have William Costigan (Leonardo DiCaprio), a police trainee who comes from bad blood. After a hostile discussion with Captain Queenen (Martin Sheen) and Sergeant Dignam (Mark Wahlberg) of SIU regarding his family of thugs and mobsters, they use his tainted family name to their advantage; using him as a mole inside of Costello’s crew to help bring him down. 

This is where the movie starts picking up. The cast is established, the setting is clear and the main task(s) are clear. For Sullivan, that means to subtlety sabotage SIU’s efforts and keep Costello informed, for Costigan it’s to keep SIU informed and build a case against Costello, while Costello (with Sullivan's help) must keep on evading the law, which with the help of Sullivan won’t be too difficult. One particular method Sullivan has for tipping off Costello that something is up is the line “Dad, I don’t think I’ll be home for dinner”. The line is used several times throughout the movie with the occasional garnish to add flavor to the dialogue, and also convey an important part of the message. This line is especially significant because Sullivan grew up with his grandmother, so while Costello was a mentor to Sullivan, he is also a bit of a father figure.

But while Sullivan is trying to keep Costello in the clear, Costigan is doing his best to gather compromising information to bring him down. Somewhere in the intertwined actions of the warring spies (referred to more commonly as rats in the movie), Costello catches wind of a member of his team being an informant. Much of the conversation on Costello’s end from here on in the movie shifts towards the rat and it even portrays itself in the visual aspects of the film. One scene in particular speaks to this with a sudden cut to a piece of paper that Costello is drawing a horde of rats on, before he and Costigan discuss the matter of finding said rat.

As the movie progresses and the action starts coming to a climax the lines between good and evil start to blur. Who is worse, the rat who works to bring down other criminals but helps the kingpin, or the rat who works with the police to bring down the kingpin, but actively perpetuates corruption and crime to earn and maintain the kingpin’s trust? It’s incredibly difficult to go more in-depth without giving away large plot spoilers so I’ll leave it at a line of dialogue that Costello says to Sullivan when Sullivan is a bright-eyed boy learning the life of crime, “They used to say we could be cops or criminals, what I’m saying is when you’re looking down the barrel of a gun, what’s the difference?”