A repressed memory was a memory that had been unconsciously blocked due to the memory being associated with a high level of stress or trauma. It was a controversial subject, as many researchers doubted that the memories that the subject was persuaded to remember were actually accurate. Doctor Emmett Brown believed in the repressed memory theory, and Marty McFly successfully used objects to bring back the accurate repressed memories of Edna Strickland.
In 1931, Edna Strickland stole the temporal duplicate DeLorean. She traveled back to 1876, and tried to burn down the Palace Saloon, as she had done with the Hill Valley speakeasy during Prohibition. However, the fire spread and she ended up burning down the entire village of Hill Valley, which would never be repopulated and remained barren in 1931. This brought her into a deep sense of denial, as she refused to think that she was a hooligan. She repressed the memories of the arson, preferring to think only about the present, and lived alone for decades in a house on the land that was once Hill Valley, under the name "Mary Pickford".
When Hill Valley disappeared from existance, William McFly told Marty and Doc that there used to be a town where they were standing, but that it burned down before he was born. He told them to go to the house of "Scary Mary", as she knew everything about the area. When Doc and Marty got there, they realized "Mary" was actually Edna. Doc explained that Edna was exhibiting the signs of repressed memory syndrome. Marty used various objects to jog her memories, and Edna went inside and got a newspaper, which showed that the town burned down in 1876. She was about to shoot Doc and Marty for asking too many questions, but William McFly grabbed her rife, which shot off in another direction. Doc and Marty thanked William then took the DeLorean back to 1876, to stop the arson of Hill Valley, and to bring Edna back to 1931, where she was imprisoned for the Hill Valley speakeasy arson.