She worked as the assistant to Monica Hooper, the owner of a successful architecture company. While Lisa initially appeared to be friends with Monica, Lisa actually despised working under Monica, believing her to be domineering and desiring to take over the company and gain her wealth. She began scheming to do just that when she had her husband Kevin (the film's primary villain) seduce and eventually marry Monica, who had killed Monica's husband Jonathan by driving their car off the road one night.
She and Kevin plotted to have Monica committed to a psychiatric hospital, drugging her to have headaches and memory loss and later hiring nurse Dr. Sommers to keep her heavily medicated once they have her committed to her psychiatric hospital. The pair met with lawyer Jerry (another conspirator in their plan) to confirm that Monica's estate and company were now in Kevin's name, where Lisa revealed she was also on board with Kevin's plan of raising Monica's daughter Maddy as their own. When Monica's parents Barbara and Harry came to take Maddy home with them, Lisa retrieved Maddy from their car as Kevin took the two captive, planning to kill them later to keep them quiet.
However, as Kevin was driving home from the hospital after explaining his entire plan to a seemingly catatonic Monica, she called him as he was on the phone with Lisa to reveal she was lucid and had recorded his confession. After confronting Kevin, Monica questioned Lisa on her betrayal of her, with the evil Lisa proclaiming she was better than Monica as a businesswoman, wife, and mother and was simply replacing her. Shortly after ending the call with Kevin, however, Lisa was confronted by the police and led out of Monica's house in handcuffs.
Later, a few moments after Lisa's defeat, Kevin chased the escaping Monica by car, where Monica vowed that Kevin will pay for his crimes, and Kevin ultimately does pay the highest price for his evil deeds, when a police car chased Kevin and rammed into his car, causing Kevin to lose control and plummet screaming to his death off a cliff into the river below.