Harold Farnum was a weak pansy in love with his boss' daughter Sheila. He proposed to her at an auction they were sent to by her father and she mocked him for even considering it. While Harold dreamed about not being such a wuss, Sheila asked him to interrupt the auctioneer and ask him when the Gramercy lot, which their father had sent to bid on, was up. His attempt to ask the auctioneer resulted in him accidentally buying a mirror that belonged to 11th century knight Sir Guy de Montfort. The cowardly Harold was too scared to tell the auctioneer that he wasn't actually bidding on the mirror and bought it even though it would cost him several weeks of pay. Sheila made it clear that this kind of behavior was the reason why she could never marry him and why her father treated Harold like a fool.

   Harold hung up the mirror in his room and suddenly saw the reflection of a room in an old castle inside the mirror. Harold's mind was then inadvertently transferred to 11th century knight Sir Guy and watched through his eyes how Sir Guy protected Lady Anne from Sir Wolfran and courageously fought him and his underlings. He received a cut on his forehead during the fight and turned to the mirror to examine it, but Harold's mind was back in his own wimpy body and he had no cut on his forehead.

   Sheila arrived at Harold's place with a portrait of Sir Guy she had bought at the auction because the famous knight resembled Harold, which she considered humorous. Harold, instilled with the courage of the famous knight he had possessed for a short time, then kissed Sheila and told her that she was going to marry him the next day after he demanded a business partnership from her father that night. Harold then smashed the mirror because he didn't need it anymore. Sheila was surprised, but Harold told her not to question his actions because he was the man of her dreams and loved her.

--Mystery Tales I#41/3

Note: Though it was never outright stated in the story, it is possible that the reason for Farnum's resemblance to Sir Guy was that they were related.

   Sheila was the daughter of Harold Farnum's boss. She rejected Harold's marriage proposal at an auction they were sent to by her father because she wanted to a knight in shiny armor and Harold was hardly that. She told Harold to ask the auctioneer when the Gramercy lot their father had sent them to buy was up, but Harold screwed it up and accidentally bought the mirror of famous 11th century knight Sir Guy de Montfort because he was too scared to correct the auctioneer for thinking Harold was bidding on the mirror. After the auction Sheila made it clear that this kind of wimpiness was exactly the reason why she would never marry him and why her father treated Harold like a fool.

   Sheila returned to the auction after Harold had left with the mirror and bought a portrait of Sir Guy de Montfort because the famous knight resembled Harold, which she considered humorous.

   She visited Harold to show him the portrait and mock him, but with newfound confidence Harold kissed her, told her that he would demand partnership in her father's business that night and that they would get married the next day. He then smashed the mirror he had bought at the auction and Sheila had the audacity to question his actions, which he made clear she should not do because he was the man of her dreams and loved her.

--Mystery Tales I#41/3

Anne was an 11th century lady living at the same castle as Sir Guy de Montfort. She was harassed by Sir Wolfran while Sir Guy was away, but the noble Sir Guy returned in time to protect her. Sir Wolfran ordered his men to attack Sir Guy, but they were easily defeated by Sir Guy. Anne noticed a wound on Guy's forehead from the battle while Guy held her in his arms.

--Mystery Tales I#41/3

Sir Wolfran lived at the same castle as Sir Guy de Montfort and harassed Lady Anne while Sir Guy was away, but to his surprise Guy had already returned and stopped Wolfran's harassment of Anne. Sir Wolfran was angered by Sir Guy crossing him once again and ordered his men to attack Guy, but the famous knight easily defeated Wolfran and his men.

--Mystery Tales I#41/3

It was in Sir Guy de Montfort's possession during the 11th century and either always was a magic mirror or became imbued with magic at some point. In the modern era of the 1950s it had the power to not only serve as a window to the past, but also temporarily transfer the mind of whoever was looking at it into the physical form of 11th century knight Sir Guy de Montfort. It is unclear whether the person temporarily inhabiting the form of Sir Guy actually controlled his actions or just had the opportunity to watch the world through his eyes for a short time.

--Mystery Tales I#41/3

Note: There is a slight possibility that the reason Farnum was transferred by the mirror to Sir Guy's form was that Farnum was actually a descendant of Sir Guy.