The Mysyerious Stranger requires preparation prior to having sex to be ready for it. To complete the mysterious stranger you need to start having sex with your girl by a window. Also, you must be fucking her from behind so she is not looking at you. Then, in one swift motion you will have to pull out your dick, and have your buddy, who has been secretly in the room the whole time without the girl knowing, jump in and take over for you. Then, while he continues fucking her, you run outside and wave at your girl through the window and enjoy the suprised look on her face when she sees you.
"ohh baby you like that?"
"yeah i like thatttt"
"hey that guy outside the window looks just like y--- AHHHHHHHHH"