In a game of dare, a sister challenges her brother to watch her pleasure herself with a strange bottle she's found. He agrees, but as she plays with it, her eyes locked onto his growing bulge, he can't help but join in. She lubes the bottle with her saliva, teasingly offering it to him. He takes it, feeling her heat, and soon replaces it with his own pulsating member. They fuck feverishly, the bottle forgotten, until they both climax in a messy, taboo embrace.