They are apparently made mostly of plastic. ("Not So Bad A Dad") The name of the doll refers to a MacGuffin, a common item in TV shows and film whose single with the sole purpose is to drive the plot of the episode forward, either directly or indirectly.
Candace Flynn had a Mary McGuffin doll with light brown hair and a green dress as its apparel. She presumably cherished it dearly. She was distraught upon finding out that stepfather Lawrence Fletcher accidentally sold it in their garage sale for a dollar. Upon this moment, Candace embarked on a city-wide search for the doll with the help from her brothers, Phineas Flynn and Ferb Fletcher.
It turns out that the local evil scientist Heinz Doofenshmirtz was the person who bought the doll at the garage sale, as he intends to give it to his 16-year old daughter Vanessa Doofenshmirtz, who had asked for a Mary McGuffin doll when she was seven. For him, finding it marked the end of years of relentless searching over the internet and other garage sales.
Upon receiving the doll, Vanessa was at first disappointed, since she is a little too old for dolls and that the last thing she expected was a brand new car. However, upon hearing that her father spent years trying to find the discontinued doll, Vanessa starts to appreciate it very much, finally seeing how much Heinz tried to show his affection for her in spite of his reputation as an evil scientist. However, it was mistakenly packaged into a donation box and sent off to a Charitable Charities department in Danville by her mother Charlene Doofenshmirtz, prompting Vanessa to head over to the place to retrieve it back.
When Vanessa found it on top one of the shelves, she was unaware that Candace had found it at the same time and had the same mindset. The two teenagers briefly fought over it before it fell into the hands of Sally, who now wanted possession of it as well.
Realizing that it would be wrong to just swipe the doll from Sally, Candace finally decides to forfeit the doll, saying that she had outgrown it. Vanessa almost followed suit, but in the end, she decided it had too much sentimental value for her to just give up. So without hesitation, Vanessa snatched the doll from Sally and walks back to her home with it, leaving Sally to cry. Vanessa would later phone her father about the event, and he sees this as a milestone in their relationship, feeling very proud at Vanessa for doing something evil.
As of currently, the doll belongs to Vanessa.
- The doll is a possible parody of Lalaloopsy dolls because of the appearance.