Kavanaugh has been described as the toughest adversary the Team ever faced. He has degrees in both criminology and psychology, and frequently played mind games with people under surveillance to find out what their motivations are. It was implied that his reason for taking the case revolved around Assistant Chief Roy Phillips' belief that it was an expedient time to prosecute police corruption and spin the news to his advantage. Kavanaugh thus hoped to advance his career by building a case against the Strike Team. Series creator Shawn Ryan has commented how Kavanaugh's appearance changes over the course of Season 5 as a result with his obsessive pursuit of the Vic and The Strike Team. He loses weight, tidies up his appearance, and loses his scruffy excess facial hair.
Kavanaugh was once married to a woman named Sadie, but the two were divorced as a result of her severe mental illness and his own unwillingness to continue taking care of her. Despite this, he continued to wear his wedding ring and, for a long time, was still in love with her. At one point, Sadie faked a rape incident as a means of bringing her and Kavanaugh back together. When Kavanaugh learned the truth, she pleaded with him to bend the rules for her, saying that she loved him and wanted to come home. Kavanaugh refused, revealing that he had long ago chosen his career over his marriage. He arranged for her to be charged with filing a false police report and remanded to a State psychiatric ward. An enraged Sadie later killed whatever hopes of reconciliation still remained by engaging in a torrid sexual encounter with Vic Mackey and lewdly taunting her ex-husband with the details.
Kavanaugh's initial investigation involved a meeting with Councilman David Aceveda, who grudgingly agreed to assist him in hopes of sending Mackey to prison. He informed Kavanaugh of Detective Terry Crowley's deal with Feds and his own belief that Mackey murdered him in retaliation. He also described Vic's teammate, Curtis "Lem" Lemansky, as "the conscience of the Strike Team."
Kavanaugh began by targeting Vic's ex-wife, Corrine Mackey. However, Corrine became suspicious of his behavior and later identified him as a police officer. She would assist him in his investigation as per instructions from Vic. Lem, caught stealing heroin during a bust, reluctantly agreed to wear a wire to uncover evidence on Vic. Through this, Kavanaugh discovered a plot to traffic pharmaceutical drugs to the Russian Mafia and take money under the table. When it appeared the deal had gone through and that Vic had committed murder, Kavanaugh's task force swooped in and handcuffed the team. However, instead of a crime scene, Kavanaugh discovered that he had unwittingly destroyed a top secret sting operation. He also learned that the Strike Team's bugged room had been discovered and used to feed him false information. In addition, he also learned that Lem had taken care to leave the room or otherwise not have any incriminating evidence recorded. After listening to Vic and the Team boasting about how, "We'll beat Internal Affairs next year," an outraged Kavanaugh privately destroyed his hotel room.
Now even more determined to put the team away, Kavanaugh set up shop in the Barn, commandeering the Captain's office and plastering it with graphic photos of the Terry Crowley murder scene. He also tried to coerce a confession from Vic by forcing him to revisit the Crowley murder scene and describe what happened in front of Terry's weeping brother. Vic was unfazed, however, and calmly stuck to his original story.Kavanaugh then widened the investigation to include Councilman David Aceveda for alleged collusion with Mackey and The Strike Team. During a raid on a Salvadorian hand grenade factory, Lem heroically saved Kavanaugh from an active grenade tossed in his direction by throwing it away seconds before it exploded. Kavanaugh responded by continuing to pressure Lem into testifying against the Strike Team. Lem indignantly refused to turn his back on his fellow cops as Vic confronted Kavanaugh's cold treatment.
However, the raid had come shortly before Kavanaugh's meeting with his wife. Vic and Lem entered the security monitor room to have a private conversation, only to see via the monitors, a rather private conversation between Kavanaugh and Sadie, the Lieutenant's distraught, mentally ill ex-wife. Viewing their disturbingly private moment on closed-circuit television, Vic remarked that they had found Kavanaugh's weakness. Kavanaugh eventually noticed that the security camera in the room was turned on. Realizing that his conversation with his wife was being observed by his enemies led him to rush out of the interrogation room in a rage. He found Vic and Lem tiptoing out of the observation room. The now enraged Kavanaugh realized that Lem had never been helping him, he ordered him to be arrested for possession of heroin with intent to distribute. As Vic and his fellow officers watched in horror, Lem was handcuffed by Julien Lowe, placed in lockup with other criminals, processed, and sent to a detention center.
Shortly thereafter, Vic paid a visit to Sadie's residence. Originally intending to milk her for information, he changed his plans after Sadie sexually propositioned him. After they both lewdly taunted Kavanaugh with the X-rated details of their encounter, the Lieutenant's former scorn for Vic Mackey gave way to a consuming hatred that blinded his judgment and caused him to discard what was once a very strict moral compass.
Enraged, Kavanaugh began to break the law himself. He tried to follow suit by seducing Corrine Mackey, but she rejected him in disgust. Kavanaugh then tried to rape her, but could not bring himself to go through with it. He viewed this as a moral defeat by Mackey.
Kavanaugh also secretly made a deal with imprisoned drug lord and cop killer Antwon Mitchell, hoping to use his knowledge of the Strike Team in order to destroy them. He also had the Strike Team's assets frozen, as well as those of Vic's ex-wife, Corrine. Although this move was meant to prevent them from paying Lem's bail, Vic arranged for it to be paid anyway by the Team's old friend, Smitty, using stolen drug money.
Despite all of his maneuvering, Kavanaugh's investigation was destroyed by external factors. In a move to seal the other three members of the Strike Team off from prosecution, Lem plead guilty to theft under color of authority, agreeing to serve 18 months before parole of a 5 year sentence. The Department and the District Attorney, who had both lost their patience with Kavanaugh due to rising complaints from the Barn, the expense of paying for his Strike Force, and the lack of progress in the case, decided to accept. Lieutenant Kavanaugh, stressed out and humiliated by the strain of chasing a much smarter opponent, came apart at the seams. He angrily protested to Assistant Chief Roy Phillips, saying that that Vic Mackey "is pissing all over us. And we just smile, and lick it up?" Phillips interrupted him and told him "that's enough", but Kavanaugh went on anyway and dramatically pointed to the crime scene photos of Terry Crowley and screamed,
"Vic Mackey kills cops! He... he deals drugs! He beats suspects! You know what he did yesterday? He... he screwed my ex-wife with the sole purpose of making this investigation seem like a personal vendetta. He may have assassinated a gang leader. And that's just all in one day. I wonder what he's going to do today. I wonder what he's going to do tomorrow. This guy... this guy is just pissing... he's pissing all over us. He's pissing on you. What does it taste like? Chief, what does it taste like, 'cause you know what, it tastes like piss to me."
However, Kavanaugh was ordered to complete his investigation in 48 hours, and find someplace else to do the paperwork.
Then, however, Lem did not report for processing, and Kavanaugh was overjoyed to be given one final chance to crush his nemesis. Correctly presuming that Vic was harboring Lem, he attempted to have the Strike Team arrested for aiding and abetting a fugitive. He had the Team followed at all times and even persuaded Aceveda to leak to false information to Vic, saying that Lem had revealed the Strike Team's involvement in the Armenian Money Train Robbery.
Kavanaugh also deliberately sabotaged Vic's arrangement with Antwon Mitchell, which was meant to keep Lem alive while serving his prison sentence. He confronted the imprisoned drug lord, enraging him to the point that Antwon swore he'd have Lem killed no matter which prison he was sent to.
Kavanaugh had the Team trailed when they were to meet Lem, but was unable to follow all three of them. His tails were easily shaken, but Kavanaugh soon received a phone call that Lem had been blown apart by a hand grenade, presumably by the Salvadoran drug cartel led by Guardo Lima. When the body was discovered, Kavanaugh and Mackey were face to face once more. Kavanaugh was certain that Vic had again murdered a fellow officer, and demanded to know,
"Are you happy now?"
Vic, completely devastated by a murder he had nothing to do with, tackled Kavanaugh with a scream of outraged fury. The two enemies brawled across the crime scene until their fellow officers pulled them apart. As Kavanaugh watched Vic Mackey walk away, his eyes burned with a vicious hatred. Meanwhile, Vic gestured toward Lem's body and snarled,
"We're gonna find whoever did this, and we're gonna kill 'em."
Kavanaugh's final charge
With Lem now dead and his career and reputation soiled, Kavanaugh was ordered by Chief Johnson to leave the Barn and allow the investigation into Lem's murder to proceed. Kavanaugh pleaded for time to look into Vic's possible involvement, as part of a potential motive for Lem's murder. Assistant Chief Phillips grudgingly granted him 24 hours to write a report for consideration. However, upon arriving at the Barn, he immediately told Vic,
"You murdered two members of your Team!"This instantly set off another brawl. The fight was broken up, but was witnessed by Phillips and Captain Wyms. Kavanaugh was ordered to leave the Barn immediately, his investigation apparently over. Desperate to destroy the man he now viewed as a two time cop killer hiding behind a badge, Kavanaugh informed Claudette and Phillips that Vic had hired the Salvadoran drug dealers who killed Lem, using Emolia Melendez as a go-between. This granted him a temporary reprieve.
After Emolia agreed to follow the story in exchange for witness protection, she testified to Claudette and Phillips about the false details of Vic's involvement. Kavanaugh was authorized to investigate Vic's involvement, and Kavanaugh began by ordering officers to await Vic at his home to take him in for questioning.
Vic paid a visit to the Lieutenant's residence, taunting him by saying,
"You are entering a whole new playing field, my friend. I hope you know the rules."Kavanaugh responded by breaking into Vic's house and planting several pieces of "evidence" to further solidify the murder charge.
However, Kavanaugh soon sensed that not everyone believed his story. After hearing Emolia's testimony, Dutch expressed skepticism about her story and requested further corroborating evidence. He later suggested that the "evidence" found in Vic's house was simply too good to be true. In an attempt to protect his case, Kavanaugh requested that Dutch be removed from the investigation, citing that Dutch's past relationship with Vic's ex-wife may have resulted in a conflict of interest.
Then, however, Corrine Mackey learned of the charges against her ex-husband and angrily confronted Kavanaugh, formally accusing him of sexual assault. When Claudette asked Dutch for his opinion, he expressed skepticisim about Corrine's allegations, suggesting that the rape attempt only happened in her own mind. He further spoke of his growing doubts about Kavanaugh's case, questioning whether Vic would murder a close friend under any circumstances. Claudette told him of her belief that "only the truth can save this place." Together, they began to take a good hard look at Emolia's story.
As Claudette tore into Emolia in the interrogation room, Kavanaugh realized what he had become. He stopped Claudette and confessed to planting the evidence.
"I framed a guilty man,"he said, laughing at the irony of it all.
Kavanaugh was immediately stripped of his badge and imprisoned for the many crimes he had committed. He later plead guilty in exchange for a lesser sentence.
Taking a break from his mission to wreak bloody revenge on Lem's killer, Vic paid Kavanaugh a visit in prison. Although Vic taunted him about having lost both his moral compass and their private war, Kavanaugh remained unfazed. He responded that he was now at peace and finally free of Vic's corrupting influence. He added,
"One day the Universe will take out its trash."