This fan theory about Will Ferrell's film Elf is blowing fans' minds

Will Ferrell in the popular Christmas film, Elf. Photo / Supplied

A fan of the Will Ferrell movie Elf has put forward a bizarre theory that suggests his character Buddy actually turns up in another of the comedian's movies.

The fan, who only goes by Trent, put forward the theory that Buddy is actually Ferrell's character in Step Brothers, albeit having had his name change to Brennan.

What if Buddy's step mom in Elf divorced James Caan, got custody of Buddy, renamed Buddy to Brennan, remarried and that's Step Brothers?

— Trent (@BarstoolTrent) December 6, 2016

Mary Steenburgen plays Ferrell's mum in both movies, well actually his stepmother Emily in Elf.

"Some people responding to my theory tweet said: 'Why would Emily get custody of Buddy when Walter (James Caan) is his biological father?' The answer to that is obvious," Trent writes.

"During the divorce hearings Walter likely said Emily could have Buddy cause he didn't want him in the first place and she clearly did want him. She's more fit to be his guardian."

Will Ferrell with John C. Reilly in Step Brothers. Photo / Supplied

Under Trent's theory, Emily divorces Walter, changes Buddy's name to Brennan and then remarries in Step Brothers.

Step Brothers was shot five years after Elf, so the timeline makes sense. He also points out that the brother of Ferrell's characters in both films look similar.

"One of the strongest pieces of evidence. Both Buddy and Brennan are amazing singers and their pipes are proudly displayed in each movie," he writes.

"It's mentioned in Step Brothers that Brennan recently worked at PetSmart which means he loved animals enough that he wanted to be around them everyday for his job. You know who else loves animals? Buddy the Elf."

"Both Buddy and Brennan cannot function as adults. It's the main issue in both flicks," he writes. "When Buddy's step mum in Elf asks, 'How long will you be staying with us?' Buddy says 'forever' and he means it.

"Brennan refuses to get a job and refuses to move out of the house because he's actually Buddy the Elf and wants to live at home forever," he said. "So there you have it. Elf is the prequel to Step Brothers and you cannot convince me that's not true."

I can see the resemblance! pic.twitter.com/jGTQCDzrN9

— Mike Celly (@MikeCelly) December 6, 2016

Trent's theory has been retweeted almost 5000 times with people saying their mind is blown by the theory.

It actually makes a lot more sense then the fan theory that Sandy (Olivia Newton John) from Grease died at the beginning of the movie and that the rest is merely her fantasising about what life might have been like had she lived.

. @BarstoolTrent pic.twitter.com/lZGKeNaLdP

— Armin Gierow (@armingierow) December 6, 2016

pic.twitter.com/Q7ouxTZVPE

— Ryan Dugan (@DuganRyan8) December 6, 2016