Arthur Mitchell is portrayed as an unassuming Antarctica and family man — who for 30 years has been living a double life as a serial killer. In the series, FBI agentFrank Lundy dubs him the "Trinity Killer" because of a recurring pattern of three killings. The series shows that Mitchell's first murder is of a young woman in a bathtub, killed by slicing her femoral artery with a straight razor while putting her in a choke hold, and holding up a small mirror so that he can see her face as she dies. His second victim is a married mother of two, whom he kidnaps. He forces her, by threatening her husband and children if she does not comply, to fall to her death from a carefully selected abandoned building. His third victim is a father of two, whom he bludgeons to death with a hammer. At each site he places a small sample of his sister's ashes, and arranges the victim's arm to point at the ash. He has repeated this cycle in different cities all over the United States. Frank Lundy's superiors are skeptical about the existence of the Trinity Killer, leading him to launch a private inquiry into the murders that eluded him during his FBI career. It is later discovered that, before these three, the Trinity Killer also kidnaps a young boy whom he dresses in cowboy pajamas and convinces to play with a train set in a recreation of his childhood. Following this, he will bury the boy alive in cement at a Four Walls' building site. These killings were not connected to the pattern until shortly before Mitchell's death, because the boys were reported as missing rather than dead. At first, Dexter Morgan is fascinated by Mitchell's efficient killing methods and apparent ability to balance his familial responsibilities with his secret life as a serial killer.
Family
When Mitchell was 10, he spied on his older sister Vera taking a shower; when she saw him, she was so startled that she slipped and fell through the glass door, slicing her femoral artery and bleeding to death. His parents blamed him for Vera's death. His mother committed suicide two years later by jumping out of a window. His father, already a heavy drinker, became an alcoholic and frequently beat his son during his drunken rages. Ultimately, his father was bludgeoned to death with a hammer - it is suggested by Mitchell himself. Mitchell re-enacts the death of his family in his murders: the young woman represents his sister, the married mother of two represents his mother, the father of two represents his father, and the boy represents Mitchell himself. Mitchell lives with his wife, Sally, and his teenaged children, Jonah and Rebecca. He has another, older daughter by a previous relationship: Christine Hill, who witnessed one of his murders as a child. He treats her like a shameful secret, forbidding her from contacting him and keeping her existence hidden from his other family. She tries unsuccessfully to win his love, going so far as to commit murder for him; she kills FBI agent Frank Lundy and wounds Dexter's adopted sister Debra to protect her father from the police. Her actions have the opposite of the intended effect, however: When he learns what she has done, Mitchell is enraged and tells her he wishes she had never been born. She commits suicide shortly afterward.
Story progression
Episode 37
In season four, Mitchell begins the cycle again, killing his victims at the original murder sites. Mitchell punishes himself after each murder and soon enough he kills him self: after killing the woman in the bathtub, he takes a shower in scalding hot water; after forcing a mother to fall to her death, he provokes a drunken man into beating him up.
Episode 40
agent Frank Lundy comes out of retirement to hunt Trinity down, but is shot and killed just as he begins to close in on him. Lundy had theorized that Trinity was a loner who had no life outside killing. When Dexter goes to kill Trinity, he discovers that Trinity is a husband and father, as well as a prominent figure in a charitable Christian homebuilding organization. He uses this organization to travel around the country to cover for his murder spree, and as a dump site for the first victim in his cycle.
Episode 43
Dexter is conflicted over whether to kill Mitchell or learn how he makes his family life work. Dexter, under the alias of Kyle Butler, befriends Mitchell and soon learns that, unlike Dexter, Mitchell apparently has no problem being himself with his family or expressing affection. He also witnesses Mitchell's strange, unpredictable personality; the same man who murders people with uncommon brutality starts crying when he and Dexter hit a deer, and is appalled at the idea of putting it out of its misery.
Episode 44
As Mitchell plans a trip to Tampa to build a house, Dexter concocts a plan to go to a meteorological convention to establish an alibi so he has a reason to be in Tampa as well. Mitchell reluctantly allows Dexter to tag along when Dexter claims he's done something terrible that only Mitchell can help him recover from. Dexter, hoping to redeem his murder of an innocent man, plans to murder the decidedly guilty Mitchell along the way. During their road trip, however, Mitchell pushes Dexter to admit his terrible deed, and Dexter claims to have killed someone in a hunting accident. This admission greatly affects Mitchell, who sees it as a sign of kinship. He takes Dexter to his old home and tells him that when he was 10, he startled his sister while spying on her in the shower; she fell and broke the glass door, slicing her femoral artery and bleeding to death. His mother later committed suicide by leaping off a building, leaving him in the care of his abusive father; Dexter conjectures that Mitchell bludgeoned his father to death, accounting for the third victim. For the remainder of the trip Mitchell exhibits sudden mood swings and irrational behavior. He keeps telling random people about his family members' deaths and stresses the need for confession, to the point that Dexter is actually worried that Mitchell might tell people about Dexter's previously mentioned murder. One morning, Dexter sneaks into Mitchell's room, intent on killing him, only to find him gone. Dexter tracks him to a construction site where Mitchell attempts suicide. Dexter reluctantly stops him, wanting to kill Mitchell himself. Dexter contemplates letting Mitchell fall to his death, but as he's about to let go, workers at the site come to help Dexter save him. Afterward, Mitchell has a newfound zest for life, believing God sent Dexter to save him.
Episode 45
As Dexter gets to know Mitchell, he learns that his would-be mentor is not the loving family man he appears to be; Mitchell dominates his wife, beats his son and keeps his daughter virtually imprisoned in her room. During Thanksgiving, Mitchell's son, Jonah, lashes out at his father, destroying his homebuilder's plaques and smashing his sister's urn. In a fit of rage, Mitchell almost strangles Jonah, but Dexter interferes, dragging him into the kitchen. Just as Dexter has Mitchell at his mercy, however, Mitchell's wife and daughter rush in to stop him.
Episodes 46 and 47
Dexter witnesses Mitchell kidnap a child, but is unable to stop him. Eventually Dexter tracks him down as he's about to bury the boy alive in drying concrete. Dexter saves the boy, but Mitchell escapes. After Mitchell learns from a news report that the boy is alive, and that Dexter didn't turn him in, he begins to become suspicious. As he tries to find Dexter he kills a man named Kyle Butler, and ultimately follows Dexter to the Miami Metro Police Department. There, he discovers Dexter's real name.
Episode 48
In the last episode of the season, "The Getaway", Dexter subdues Mitchell and prepares to kill him in the room where Mitchell had held the kidnapped child. Mitchell believes Dexter was sent by God to kill him, and claims that he had tried several times to stop killing, but found that he could not change his "path". He predicts a similar fate for Dexter: he tells Dexter "It's already over", the same thing he says to his victims before he kills them. Dexter then kills him with a hammer. It is revealed in the final scene of the episode that Mitchell killed Dexter's wife, Rita, in the bathtub of Dexter's house, and that he left Dexter's son Harrison in her blood, mirroring the childhood trauma that put Dexter on the path to becoming a serial killer.
Episode 52
Miami Metro detective Joey Quinn suspects Dexter of killing Rita, and he connects the dots as to the identity of Kyle Butler. Anxious to further his investigation, Quinn requests permission from Captain Maria LaGuerta, but she denies his request and orders him to leave Dexter alone. Undeterred, Quinn finds Jonah Mitchell amidst being relocated into witness protection with the rest of his family, and tries to show him a picture of Dexter so that Jonah can confirm whether or not it is "Kyle Butler". However, one of the FBI convoys manages to intercept Quinn's attempt before he succeeds, and so Quinn's query remains unanswered.
Episode 67
In the sixth season, Dexter hears news about the Mitchell family, who are now living in Nebraska: Mitchell's wife, Sally, and daughter, Rebecca, have been murdered in the same way as were Mitchell's victims. Dexter deduces that Jonah is the killer, and hunts him down. At first, Jonah claims his father killed them both, but later gets into a fight with Dexter, seemingly wanting to die by Dexter's hand. Confused, Dexter gets Jonah to confess that Rebecca actually committed suicide because their mother kept blaming the children for Mitchell's capture; upon finding her, Jonah flew into a fit of rage and killed his mother, bludgeoning her in the same manner that his father killed his victims. Dexter tells Jonah to forgive himself, and leaves.
