Elliot and Wesley take the girls on a well-deserved island vacation. Everyone seems to enjoy the sun, sand and surf -- except for Bridget. Despite being infused with the DNA of a finless porpoise, she's never learned to swim. This changes when the mews meet a little girl named Cassandra who has a fear of the water because she almost drowned a few years prior to the episode. Bridget and Cassandra try to teach each other to be brave and to try swimming again. Meanwhile, the Cyniclons Sardon and Tarb are on the ocean floor, culturing new Predasites.
As Bridget broods about her failures (and about her almost drowning), Elliot tells her that she'll find the bravery within her. Sure enough, when the Predasites attack and kick up a storm, Mew Bridget dives in after Cassandra, who is trying to rescue a young boy. She sees a strange light...and, suddenly, her legs turn into a fin. She is able to breathe underwater like a mermaid...and anyone she holds can do the same. As Mew Bridget leaps out of the water, the light weakens the Predasites, allowing the other girls to "Mewtilate" them.
On the last days of the vacation, Bridget is swimming quite well alongside Cassandra, while Elliot and Wesley are left to wonder if their Mew Mews have stronger powers than before.
There were some cuts in this episode. Some of them are:
- Renee stripping down into her bikini with Corina sparkly face shot.
- Kikki and Renee talking about their "Red Data" animals.
- Kikki listing what each animal the 4 Predasites were made of.
- They replaced the skeletons shown while Mint and Pudding were getting shocked by extending the first part when their actual bodies were shown
- Iruka's named is changed to Cassandra.
- When Corina mentions that she has the DNA of the Blue Lorikeet and can sing karaoke, the Japanese version states that Corina can't sing.
- The episode title is a pun on the phrase "a girl with a purpose".