The Forbidden Barn is a barn located at the Movementarian farm.
People who joined Movementarianism were told that there was a spaceship inside the barn that would take them to a planet called "Blisstonia." Nobody was allowed to enter, but The Leader was occasionally seen emerging from the barn being driving around in his Rolls-Royce.
When Homer tried to expose The Leader as a fraud, he opened the barn doors, intending to show people that there was nothing inside. To everyone's surprise, there really was a spaceship. However, when the spaceship started to fly, it was exposed as a fake when it fell apart and The Leader was revealed to be inside, attempting to escape by flying away with his followers' money.