A Siren arrives in San Francisco, looking to cause trouble for couples. After she kills the husband of a future Whitelighter, she sets her sights on Phoebe and Cole, Piper and Paige have to find a way to keep them from dying. That proves to be tricky when, following an argument between Leo and Piper, their unborn baby uses its magic to make them swap powers, in an attempt to walk a mile in one another's shoes. Cole continues in his quest to prove to Phoebe that he's no longer evil.