In Howling V, Mary Lou Summers was part of a group of random people who all came together in a castle in Budapest that had been closed for 500 years. The host of the gathering, Count Isht Van Bezeli, revealed that they were all descendants of a family that occupied the castle back then and had all committed suicide, while a baby who carried the werewolf curse survived. Because of this, one of the descendants is a werewolf, and he or she had been killing the others one by one. In the film's climax, Mary Lou was torn between shooting the Count or her own husband, David, after the Count suggested that David could be the werewolf. However, Mary Lou spares her husband and kills the Count, and she embraced David, who assured Mary Lou that werewolves don't exist. Mary Lou turned villainous in the final scene by flashing an evil smile as soon as the full moon appeared, revealing herself as the werewolf and killing David (offscreen).Mary Lou resurfaced in Howling: New Moon Rising under a different name, Cheryl, and continued her villainous activities as a werewolf. Ted Smith, an Australian drifter, was accused of being the werewolf, and in the film's climax, Cheryl encounters Ted and takes her to her home for safety. It was there, however, that Cheryl revealed herself as the true villainess, Mary Lou Summers, and she boasted to Ted about her evil plan to frame him as the werewolf. The film indicated that an actual Cheryl existed, but Mary Lou killed her and assumed her identity. Mary Lou planned to kill Ted with her shotgun, but when she fired, it was revealed that the gun was loaded with blanks. The blanks were part of a set up by Ted and a local detective, who knew about Cheryl's true identity.
At that moment, the evil Mary Lou transformed into a werewolf after opening the curtains and seeing the full moon in the night sky. After her villainous transformation, Mary Lou bursted through her front door, but as she did, several townspeople shot and killed the villainess with silver bullets.