Lucy Underhill is the daughter of teacher David Underhill, but as her backstory revealed, she was at the library with her father as he was being accused of kidnapping young Maddy Beauregard--an accusation made by Maddy's mother, Claire, who went into a panic looking for Maddy and saw David as he was taking Maddy outside to the front office. David maintained his innocence, but the accusation resulted in him losing his job as teacher due to being labeled a pedophile. Since losing his job, David and his wife Vanessa argued constantly, and later on, Lucy witnessed her father commit suicide, doing so by shooting himself in the head.
Lucy blamed Claire Beauregard for her father's death, which also drove her mother insane, and swore revenge. At age 20, Lucy posed as the best friend of Maddy, while going by the name Gina West, and invited her to a college party. While there, Maddy was introduced to Tucker, and the two of them began a relationship, which would serve as part of Lucy's revenge plot. Lucy appeared at Maddy's home to apologize to Claire for taking her to that party, but she later listened in on Maddy and Claire's argument and took a pair of scissors from Maddy's room. She later went to a strip club and paid a dancer named Abigail to flirt with Tucker. Once Abigail did so, Lucy called Maddy to tell her what Tucker was doing, which prompted Maddy to angrily confront Tucker.
Lucy was present when Claire confronted Tucker regarding his relationship with Maddy, and later went upstairs to Tucker's room. She revealed to Tucker that her name isn't Gina West, stating that the real Gina West is in New York and returned to LA for a visit. After a slight distraction, Lucy unleashed the aforementioned scissors and stabbed Tucker's chest, killing him. She later planted Tucker's blood on Maddy's car and tossed the murder weapon as she drove, planning to frame Maddy for the murder, which works; Maddy ended up arrested.
After killing Tucker, Lucy resurfaced at Maddy's home and held Sean Beauregard at gunpoint, forcing him to clean out his account and open his safe, which had cash and a passport. During the robbery, private investigator Nick Fox noticed Lucy in the home and pointed a gun at her, but the villainess shot to both men, killing Nick and hospitalizing Sean, and escaped--but not before she was shot in her arm. Her robbery was successful, as she planned to deposit the funds in a bank at the Bahamas.
Lucy was found out by Claire, after she went to the Underhill home and attempted to talk to an insane Vanessa. She was later confronted by Claire at the latter's home, with Lucy voicing her bitterness and blame towards Claire for what happened to her father. She also stated that she wanted Claire to know how it felt to witness a loved one falsely accused for a crime, which is why she framed Maddy. The two get into a fight in which Claire subdues Lucy and ties her up. When Claire reveals that she called the police, Lucy in turn reveals that she put a bomb in the fireplace. Lucy feigned remorse for her actions and begged Claire to untie her, but after Claire does, Lucy knocks her out and escapes. She watched from far as the house exploded, and believing that she eliminated Claire, Lucy laughed maniacally as she drove away. Changed into a red dress, Lucy was set to head to the Bahamas with her ill-gotten gains, only for police to arrive at her front door with Claire, who narrowly escaped the house before the bomb detonated. Lucy was handcuffed and arrested for her crimes.