Her beloved husband, who she remarried, comes home late, and she masturbates to relieve the loneliness of not being paid any attention to him... Yumi, the stepmother, plots to release the void in her heart that never seems to be filled by using her stepson Masanori. One day, the opportunity suddenly came...! Looking into the room, she sees her stepson wearing unfamiliar goggles masturbating enthusiastically. Yumi begins to masturbate as she watches her stepson, who does not notice her, and finally, unable to hold back, she mounts him and joins him in a cowgirl position! The two, who were forced by their lonely and helpless stepmother to impulsively become entangled, entered into a forbidden relationship... The mother and son, who shared a secret about their husband and developed a sense of complicity, become addicted to the thrill of having sex in secret, and before they know it, they even allow him to cum inside them, ready to get pregnant...!