Why would a smart, independent woman cheat on her husband to be in a masochistic relationship with a married man?

Meet Kat: midthirties, married with two kids, and surprised to find herself sneaking out to yet another adulterous rendezvous with Shaw. They broke things off when his wife got pregnant, but seven months later, he calls-and Kat responds.

As the countless afternoon encounters and the brief, torrid evening meetings start up once again, Kat's moral quandaries begin to subside. But when a secret rendezvous turns into a life or death situation, Kat quickly realizes that their...