Role in the film
Lizzie is first seen reluctantly filming her boss' daughter, Brace Channing at the Vandenburg launch site at night as instructed by Tim O'Hara. She comes to Tim's house to comfort him after he gets fired from his job (due to him saying to Brace that she was beautiful on the speaker). When she comes to his house, she finds Tim (who is actually Martin is disguise) and they kiss. Happily, she leaves the house hoping to talk to him again.
The next day, she meets up with Tim and talks to him about the previous night, which ends with her kissing him. Later on, Tim invites her over to his house to talk, much to her delight. She arrives dressed up but finds Brace in Tim's home instead. Believing Tim was leading her on, she angrily storms out and punches Tim when he tries to explain the situation. On her way out she discovers the spaceship and accidentally falls inside of it with Zoot. She and Zoot accidentally get shrunk down inside the spaceship by Tim soon after.
Later, when Tim is captured by SETI, he gets the scientists to enlarge the ship back to normal and Lizzie and Zoot get out. Lizzie rescues Tim, but before they can escape they are stopped by two guards. A fight breaks out, and an injured Zoot gives her a Veenox 7 nerplex (martian shapeshifting gum). In hopes of saving Tim, Lizzie chews the gum and winds up transforming into a giant alien monster, intimidating everyone. She deals with the guards by throwing one away with her tentacles and swallowing the other one whole. Lizzie jokes about her appearance and spits out the gum, transforming herself back into a human. She and Tim sneak through the lab and locate Martin to save him. The two revive Martin by reuniting him with Zoot. As they escape, they encounter the villainous Dr. Elliot Coleye. Lizzie and Tim witness Neenurt, a martian disguised as a SETI agent, defeat Coleye. Lizzie and Tim say their goodbyes to Martin and watch as Coleye transforms into a martian and get captured before escaping. At the end of the film, Lizzie and Tim decide to become a couple but are interrupted when Martin returns to Tim's house to live with them.