User blog comment:Dereviewer/Fanfic Idea/@comment-24685871-20160215161419/@comment-24685871-20160227163620

Well, actually, I already have that part figured out. See, after Gargamel locks Brainy in the cage, he goes over to his cauldron and begins mixing up a bunch of ingredients, sort of like he did when he created Smurfette, and then once the ingredients are mixed, he pours it into a vial, drinks it, and then the potion takes effect and he is transformed into the spitting image of Brainy, looking and sounding just like him. And since you're wondering how he gets to the village, here's how, Papa Smurf realizes Brainy is missing, so he sends two Smurfs to go out and look for him, Dreamy and Naughty. They soon find Gargamel (disguised as Brainy) and assume that he's the real Brainy. He tells them that Gargamel captured him and he barely escaped with his life, so they take Gargamel-Brainy back to the village. And like I said, everyone, except for Hefty, is fooled by the disguise.