 Carrie writes her column on the apparent demise of monogamy in modern New York as she sees threesomes of various kind all over the place including our quartet; she considers Mr. Big s previously unmentioned ex the third person in their bed. Miranda needs therapy being frustrated that unlike her friends she is hardly ever asked to be the third party, and places an add. Charlotte is tempted by her eligible partner to experiment with a second woman. Samantha pontificates the  guest  has the best deal, but then a wife asks her after his man walked out on her to be free for Sam...