Mary arrived home a little earlier than usual. Her last class had ended by a fire drill so Mary and her best friend Jill slipped away rather than going back inside the school. They went their seperate ways.
When Mary went upstairs she saw that her brother Roger's bedroom door was closed. She stopped outside the door and listened closely. She heard strange sounds coming from her brother so she assumed, correctly, that the sixteen year old was masturbating. Mary smiled as she ran her fingers along the crotch of her pants making herself feel so nice.
She made another assumption that her brother probably hurried home from school to have a good wank before she came home. Her plan was hatched at that moment. She left the house to walk around the neighborhood killing time while she finished planning on just how to catch her brother with his pants down.