You glance around the aisles looking at costumes and other junk lying around, when you come across a lonely pair of men's underwear just lying there on a hanger. You examine it out of curiosity, it's just a pair of whitey tighties , no price, no external tags, nothing, not even a security device like everything else was outfitted with. You do however find one tag on the elastic, "Monster Briefs XXL" written in bloody horror font. Curious you take them to the dressing room and try them on. After removing your pants and briefs, you slip them over your legs and pull them up. Almost instantaneously, you feel something incredibly orgasmic happen, feeling almost like your cock is having a massage, you moan loudly and instantly recoil hoping no one heard. You pull the elastic back and find a massive hairy organ replacing your old one. You stroke it and feel it tent up against your new underwear, you become more and more aroused with every stroke, you're sold! You slip on your pants taking care to notice your new massive bulge, forgetting your old pair im the dressing room.