Not much is known about Billy except that he was born on November 7, 1984, meaning that he is 21 or 22 during the events of The Departed. At some point in school he hit a sport teacher with a folding chair. Before the events of The Departed, he was the nephew of mobster Jackie Costigan,his mother passed away from an unknown sickness and he severed ties with his uncle.
Costigan begins in the Massachusetts State Police Academy, where he befriends Trooper Brown. Before he is able to graduate, he is discovered by Captain Queenan and Sergeant Dignam, and is brought into their office.
Meeting with Queenan and DignamEdit
As he begins his interview with Captain Queenan and Sergeant Dignnam, it is revealed that his family has ties with the South Boston Irish Mob, most notably his Uncle Jackie, a deceased, yet well respected gangster. The two decide to put Costigan into an undercover operation in an attempt to infiltrate Frank Costello's crew. He serves time in jail and is placed on probation to gain credibility. Upon Costigan's release, he contact his criminal cousin and the two begin selling cocaine. Billy starts exhibiting nefarious behavior, including beating two Italian gangsters to near death, and starting a fight in Frank Costello's bar; which results in his meeting with Mr. French. His actions finally gain the attention of Costello.
First Encounter with Frank CostelloEdit
After Billy injures his hand while beating two Providence members, he encounters Frank Costello for the first time. As they goes to the backroom with Mr. French, French hits Costigan's injured arm onto a table and searches on him, saying he's clean. Frank then repeatedly hits Billy's broken hand, interrogating Billy with accusations of being a cop. Billy holds up under questioning. The next day Billy has an "interview" with Frank, and consequently joins Frank's crew.
Meeting with MadolynEdit
As part of his cover story as a disgraced policeman he must see a "court ordered" shrink. he has appointments with psychiatrist Madolyn Madden. However, he has a breakdown when is given only two pills and she orders him to leave. She meets with Billy outside to inform him that she will help him get over his distress, she then decides to transfer him to a different councellor and he asks her to meet for a cup of coffee, much to her surprise.
Finding Out Costello's AwarenessEdit
Costigan becomes aware that Costello is aware of having a rat in his crew. So, he calls for Queenan and Dignam. After briefly fighting Dignam since Dignam threatened to delete his file, he informs Queenan about Costello's awareness of the "rat" and will find out who he is. Dignam and Queenin reveal that Costello has a mole in the Special Investigations Unit.
While with Costello and his crew in a Chinese deal, Costello is texted by Colin to keep their phones off, in which Costello insists and tells his men to check their weapons and phones off. However, Costigan secretly haves his phone on and he slowly texts Queenin, informing him of the monies presence at the deal. As the Chinese leave with their micro-chips, Costello makes a remark, saying "They didn't figure we had a navy" to Costigan. Later, he has a conversation with Dignam about Costello's awareness and Costello's mole in the SIU as he goes to meet with Madolyn.
Finding Costello's true agendaEdit
Billy goes to a junkie's house as Mr. French sets off firecrackers to prevent schoolchildren and other civilians from hearing gunfire. He smashes a picture frame on the junkie's head, which wakes him up. He asks the junkie of what he does to pay. Soon, the junkie mentions Costello's involvement with the FBI. Costigan quizzes the junkie about the FBI, in which he profuses about being high, resulting in Costigan shooting the junkie in the kneecap. The junkie reveals that Costello is an FBI informant. Then, Costigan escapes with French. At Queenan's house, Costigan informs Queenan that Costello is an undercover FBI informant and that he never gets busted since he is protected from his crimes. The next day, he picks up an envelope from Frank's crew hideout. He rewrites the word "cittizins" (Fitz mispelled it) to "citizens", in which Fitz believes it's wrong. He gives the envelope to Delahunt and leaves.
He meets Madolyn at her old apartment since she moved with Colin. She reveals that she moving with Colin to Costigan and soon after the two made love.
At a pornography theatre, Costigan spies on Colin, who is meeting with Costello. As he spies on them, Costello chats with Colin about Colin's "reassignment" and Costello gives him the "citizens" envelope. Then, when Colin leaves, Costigan tails him. Colin becomes aware and he sets out to hide somewhere to ambush him, in which Billy does the same thing. after accidentally murdering a Chinese man, Colin successfully escapes.
Costigan arrives at the bar to meet Costello. As he chats with Costello about FBI relations, Costello reveals that there's a "rat" in his group, suprising Costigan. While Costigan shits his pants, Costello begins showing off his Beretta 84B Cheetah to Costigan and running drugs through the city. Frank leaves.
Meeting with Captain QueenanEdit
He calls Captain Queenan. As Queenan goes on the train, he sees Costigan, commenting that Queenan has a tail but Queenan explains he doesn't. As they leave the train, they seperate and Costigan texts Queenan to meet him at 344 Wash. On the roof, Costigan informs Queenan that Costello will find out his true identity and will kill him. Queenin requests that they will figure out what to do. As Billy receives a call from Delahunt, telling him to meet at 344 Wash. Realizing this, the two try to escape. Unfortunately, the crew are already in the building and heading up. Queenan orders Costigan to escape as he attempts to negotiate with them. As Costigan escapes, Queenan falls to his death, landing in front of Costigan. As Costigan is over Queenan's body, Fitz, Delahunt and the rest of the crew sees Costigan and orders him to get in the van, only for them to be shot at the Internal Investigations Unit officers, with a wound on Delahunt.
At the hideout, Fitz goes at Costigan of his whereabouts. As Costigan explains, Delahunt, on the couch and badly wounded, calls for Billy. Costigan goes over and Delahunt revealed that he mistakenly gave Costigan the wrong address, but Costigan came to the right one indicating that Dalahunt know Costigan's true identity, Costigan attempts to pull out his gun and kill him but Delahunt goes on asking Billy to tell him why he didn't go there before he gargles under his throat and dies. Costigan feels his pulse and leaves the hideout.
Call from ColinEdit
As Costigan is drinking in deep depression, Colin calls him from Queenan's blood-stained phone. Suprised, he packs up his stuff but he decides to contact back. As he calls Colin, Colin says that he is taking over Queenan's unit and asks for Dignam but Colin reveals that Dignam is on a leave of absence. Colin asks to meet him but Billy hangs up.
During the discovery of cocaine in a crate, Costigan sends Colin a message of Costello's crew whereabouts. At the deal, Costigan escapes as the crew are ambushed and killed in the process, after confirming he didn't sell out Sullivan to the FBI, Costello is killed by Sullivan to cover his own tracks.
Encountering Colin and the TapeEdit
Costigan meets with Colin at the SIU headquarters. As he asks Colin for a favor to find Costello's rat and arrest him. And as Colin recovers Billy's police record, revealing his information, Costigan finds the "citizens" envelope under Colin's files and he leaves. Colin comes back to his office to find Costigan gone and the "citizens" letter on show he then accesses Billy's record and erases it. Later, Costigan encounters Madolyn in front of Colin's apartment and he gives her a disc. After Madolyn discovers the disc is a conversation between Colin and Costello she silently enters the bedroom and despite Colin's plea's does not come out Sulivan thencontacts Costigan, who revealed that Costello recorded everything for evidence for the FBI and Costello's lawyer came to Costigan about the disc. As Costigan taunts Colin since he is the mole of Costello, he tells him to meet at 344 Wash at 3:00, where Queenan was killed.
Attempted Arrest and DeathEdit
At the meeting, Costigan ambushes Colin and pistol-whips him. Costigan handcuffs him and Colin reveals that he erased Costigan's record. As they are about to leave, Trooper Brown comes up the roof and draws his gun. As Brown orders Costigan to drop his weapon and step away from Sullivan, Costigan refuses and explains that Colin is Costello's rat and has evidence. As they head for the elevator, Colin's anger expresses if they land into jury. Sullivans begs Costigan to kill him, who replies "I am killing you", meaning Sullivan's career and his relationship. As they exit out the elevator, Costigan is shot in the head by Trooper Barrigan, who shoots Trooper Brown when he discovers Costigan's body. Barrigan reveals to Colin that he was also a mole for Costello and Colin kills Barrigan in response confirming his preference to a police life and desire to change.