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.
Spoilers ahead. Coverage follows the scope labeled below.
Complete story — includes the ending
Cole and Phoebe both fall under the musical hypnotic spell of the Siren, who musically lures and attracts husbands to her and enchantingly entrances their wives to follow with the intent to kill, leaving Piper and Paige to figure out how to save Phoebe. Piper, upset with Leo because he does not understand the challenges of being pregnant, and Leo, frustrated that Piper does not realize the demands of being a Whitelighter, have their powers switched magically by their unborn child.