Rosalyn Sphinx has a rare disorder that causes her to have uncontrollable orgasms that she can't stop from coming. Living with her stepbrother Lucas Frost is a challenge, since each time Rosalyn cums in front of him Lucas gets aroused. Lucas's dad, Dick Chibbles, sees Lucas looking at his stepsister with bedroom eyes one day in the living room and kicks Lucas out lest Rosalyn get the idea into her head that fucking might cure her problem. When Dick dozes off in front of the TV, though, Lucas sneaks back into the living room to be gratified with Rosalyn making the exact moves Dick feared she might.
They start in the living room, with Rosalyn sdeep throating Lucas's fuck stick until he's nice and hard. Dick is still sleeping, so Rosalyn takes the risk of peeling off her shorts and climbing aboard Lucas's hardon right on the couch to ride him.