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. The climax of the movie has Mary Lou 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 her husband, who assured Mary Lou that werewolves don't exist. After the embrace, however, Mary Lou turned heel by flashing an evil smile as soon as the full moon appeared. She revealed herself as the werewolf and howled at the moon as she killed David.
After her villainous turn at the end of Howling V, Mary Lou resurfaced in Howling: New Moon Rising under the pseudonym, Cheryl, in a California town. She continued to turn into a werewolf and commit murders, and was planning to frame Ted Smith, a drifter, for her villainous acts. In the climax, Cheryl lured Ted to her home, where Cheryl turned heel and revealed herself as Mary Lou, and was planning to kill him with a shotgun. When she fired, it would turn out that it was loaded with blanks as part of a set up by Ted and a local detective. At that moment, the evil Mary Lou turned into a werewolf after opening the curtains and seeing the full moon in the night sky. She lunged out the front door to go after Ted and the detective, but she was shot and killed by a group of people waiting for the actual werewolf to come out.