The upcoming sequel to the 1994 comedy starring Jim Carrey and Jeff Daniels will take place over a decade after the original, rather than continuing where it last left off.
"It picks up today. It's 17 or 18 years later. It's not like a month after that happened. It's today. And we explain what they've been doing all those years," Farrelly told The Huffington Post.
However, he was unable to give any concrete plot details, saying: "I'm sorry to say, that's top secret."
Farrelly added that he has yet to see the poorly-received prequel Dumb and Dumberer: When Harry Met Lloyd, but that he has no hard feelings towards the film's makers.
"You know, I never saw it. That's the truth. But, by the way, I wished the filmmakers well on that. I don't blame them for making that movie. They're young guys and the studio said, 'Hey, we want to make this movie'.
They came to us and asked, 'Do you want to make a prequel?' And we said, 'Absolutely not'. Because every 16-year-old is dumb. The idea is that these guys are 45 and they're idiots. That's what is interesting to us.
"So we didn't want to do it, but I hold no ill-will against anybody in that movie. I want to make that very clear. But we just didn't want to do it."
Dumb and Dumber 2 is set to begin shooting in September.
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Watch the trailer for Dumb and Dumber below: