The horror film Urban Legends: Bloody Mary opens with a scene from November 5, 1969. Three football players plan on having their way with their prom dates, so they spike their drinks to make them pass out. This works on two of the girls but the third girl, Mary, tries to escape. After her date catches her a fight ensues and she gets knocked out. Not knowing what to do, he locks her in a storage trunk and she dies.
It is now 35 years later, Samantha is having a slumber party with two of her friends and joking tells the urban legend of Bloody Mary. When the family wakes in the morning all three girls have disappeared. Next day, the girls reappear and claim to have no knowledge of what happened.
Mysteriously, students at the high school are starting to die under very bizarre circumstances, including football players. Each death seems to have a striking resemblance to various urban legends. Samantha decides to do some research and finds information in the school's archived paper about the prom of 1969 and the girls that were drugged that night.
One of the girls still lived in town, another one of the girls committed suicide a few years later and Mary was never found. Samantha and her brother decide to pay a visit to the one sole survivor of that night. According to the surviving girl, Grace, she believes that it is Mary's spirit that has come back to seek revenge on the children of everybody that was involved the night she died.
Grace also refuses to give them the names of the people that were involved in the incident that killed Mary. While they are trying to find clues about the people that were involved, more people are losing their lives. Now it is up to them to not only identify the potential victims but the killer as well.