You don't wake up wanting to be someone's mistress - somehow it just happens. Mistresses follows the lives and loves of a group of thirty-something friends who met at university. Katie is a doctor who has been having an affair with one of her patients. Her best friend Trudie grieves the loss of her husband who was killed in 9/11. Then there's Jessica whose anarchic lifestyle supplies lots of gossip and humour, and the happily married Siobhan - but is she really? Scratch beneath the surface of these women's lives and you enter an absorbing world of secrets and lies, passion and lust.