In order to test Jack's faith, Kate decided to fake having a twin sister by forging a letter to herself, claiming that she had been separated from her sister Linda as a baby as part of a social experiment. When Kate arranged to meet with her "sister" alongside Jack, she instead went to the meeting posing as Linda and later claimed to have chickened out of the meeting when Jack came home. She later went to the motel she was renting a room for under the guise of meeting Linda and claimed to Jack that she felt there was something off about her "sister."
All the while, however, Jack engaged in an affair with his wife's vivacious alter ego, who was a complete change from the emotionally withdrawn Kate. After learning that Cassie had caught her and Jack at her motel room, the evil Kate murdered Cassie and later broke into her house to leave a threatening note for Jack, making it appear that "Linda" was the killer and was trying to destroy Jack's life. Under her Linda persona, Kate began acting erratic towards Jack, threatening to expose his "infidelity" and insisting that she could take over Kate's life. "Linda" also revealed to him that she was pregnant (though it is later implied that this claim is untrue). She eventually suggested to Jack that they kill Kate so they could be together, with Jack agreeing to the plan when Kate (as herself) rejected his attempt to apologize for his past trangressions.
However, when "Linda" and Jack went out into an isolated forest to dig Kate's makeshift grave, Jack suddenly pointed the gun she had given him at "Linda" herself. However, when he tried to shoot and kill his "mistress," Jack found out that the gun was filled with blanks. Kate/Linda then smirked before pulling out her own gun and shooting him in the shoulder. She then knelt down to her husband, revealing her scheme and telling him that he had brought everything on himself before shooting Jack dead. Kate then returned home and removed her jewelry and makeup, fully revealing that Linda had never existed in the first place, and also that Kate had been plotting to murder Jack once she decided that he would no longer be faithful to her.