Two young Americans meet en route to studying in Spain and forge an unusually close friendship. Once back in the United States, one of the women, who is married, invites the younger to come live with her and her husband. Over time a ménage à trois develops. But that isn't what causes the ultimate rift in the friendship. Something deeper and more insidious had been at work from the very beginning. In this exploration of what underlies certain powerful friendships, the author examines the unacknowledged factors that sometimes attract us to one another—but can also make the relationship go painfully awry.