Patty Garner was part of a quintet of girlfriends who went to Harmony High School together; a group that included her close best friend Carly Newman, Erica Campbell, Rose Crenshaw, and Abby Edwards. Unlike the other four, Patty didn't have a wealthy upbringing, and furthermore, she was looking to become the school's valedictorian and also get a scholarship so that she could move on from Harmony — a city that she was unhappy in. When she found out that Abby was going to become valedictorian, Patty went into a rage, which led her to drug Abby's drink at a party that took place on the night before their graduation. As a result, Abby felt the effects and subsequently died, with her death later being labelled as the result of alcohol poisoning.
10 years later, during the film's events, Patty (now a successful lawyer) reunited with Erica, Carly, and Rose at a reunion party, where they decide to search for a capsule that contained all of their secrets as well as Abby's. Carly had suspected that Abby's death at the party wasn't accidental, as she had obtained Abby's autopsy report, which revealed that the true cause of her death was a drug overdose.
In order to keep her dark secret hidden, Patty donned a beekeeping suit and went after Carly, later burning the report in Carly's fireplace and attacking her later as she continued searching for the time capsule. While at a nail salon with Erica, Patty discovered that Erica was getting close to the truth about what really happened to Abby on the night of her death. After witnessing Erica pull the time capsule out of the pond outside Carly's house, Patty murdered her by bashing her head with a shovel. She then tossed her body down a well, which is where Carly, Rose, and Shane (who Patty allowed Carly to suspect of killing Abby) found her later on. Furthermore, the villainess placed the beekeeper's outfit in Rose's car, planning to frame Rose for Erica's death and also paint her as Abby's killer.
Patty's evil secret was revealed when the capsule was opened by Carly. Among the secrets revealed was that Abby had planned to intentionally fail her final exam and allow Patty to become valedictorian; this was a fact that Patty never knew because she killed Abby before she got the chance to tell her. Carly opened Patty's note, which revealed her hatred of Abby over her becoming valedictorian. After being confronted by both Carly and Rose, Patty finally confessed to killing Abby, claiming that her intent was to only make her sick, as well as confessing to killing Erica as well. She later fired gunshots at both women and chased after them into Carly's house, and continued firing when they tried to flee in Rose's car. One of the bullets ignited the car's gasoline and caused the vehicle to explode, killing Rose while Carly was able to escape in time.
Carly engaged in a final showdown with her evil former friend, with gardening shears as her intended weapon. During their fight, Patty viciously accused Carly of never being her friend, as she had (in her mind) valued Abby more than her. Patty's attempt to shoot Carly failed when she ran out of bullets, but she grabbed the shovel and continued fighting with Carly, knocking the shears from her possession. The brawl ended with Carly pushing Patty down onto the shears in self-defense, resulting in the latter's death.