Phoebe is running late for work, but Piper wants her to have a look at the nursery. Phoebe doesn't know why Piper and Leo are working on it now; Piper isn't due for several months. However, Piper wants to get a jump on it before the next demon attack. She also wants Phoebe's input on what color to use for the walls. Just then, Paige orbs in and a Borneo Demon shimmers in behind her. Piper blows the demon up. Paige had a potion blow up in her face last night and is now a redhead, prompting Leo to express concern about her recent "superwitch" kick. He's also worried that Piper isn't focused enough on the baby's safety, but Piper isn't willing to take a break from her Charmed duties.
In a cavern deep in the sea, the Sea Hag is confronting Mylie, a mermaid in love with a mortal man, Craig. The Sea Hag gave Mylie legs for 30 days so she could find love, but Craig hasn't specifically said he loves her. The Sea Hag is trying to get Mylie to place an auger shell over her heart, which will take away her immortality. Mylie starts to place the shell over her heart, but instead throws it at the Sea Hag and swims away.
Leo has found a baby book that Patty started for Piper--one that Piper didn't even know existed. Piper has started a baby book for their baby, whom they're certain is going to be a girl. Leo is trying to put a window frame in place even though it's too heavy for him to lift by himself. However, he's not willing to let Piper help him out of concern for the baby. Piper isn't willing to slow down even though she's pregnant; it's not in her nature. Leo then points out Piper's baby book stops at age five--when her mother died. He thinks Piper is headed down the same path as her mother unless she slows down.
The Sea Hag is still writhing in pain when a low-level demon shimmers in. He's a messenger for Necron, a powerful skeletal being who wants immortality so he can be fully alive. The Sea Hag wants Necron to be patient. Necron's minion shimmers out, and the Sea Hag raises up a downpour to smoke out Mylie.
Mylie has assumed human form and is on land in San Francisco. She pulled Craig out of a meeting, and demands to know how he really feels about her. He's about to fly to New York. Just as he's about to tell her, a storm brews and a raindrop touches Mylie's thigh, revealing her scales. Unwilling to let Craig know she's a mermaid, she runs away. She sees the billboard for Phoebe's column.
Paige walks into the office soaking wet from the rain. Mr. Cowan calls her in and tells her she's been promoted to social worker--and it means she has to get better about being on time. Paige calls up Phoebe, who is getting ready to head for court to get a default divorce from Cole.
Phoebe heads for the garage to find Mylie waiting for her. Phoebe initially thinks Mylie's just another fan, but Mylie says she knows Phoebe is a Charmed One. A splash of water from a passing car reveals Mylie's tail. Phoebe manages to help Mylie get into her car. A puddle of water rises up and turns into the Sea Hag. Phoebe drives through the Sea Hag, who briefly explodes into water before reconstituting.
At the manor, Mylie is dried off and back to human form. Paige rushes in after being called away from work. Mylie explains that mermaids can spend eternity at sea because their hearts are normally as cold as the water. However, her heart warmed up and she wanted love--which is why the Sea Hag gave her legs. Unfortunately, she's reluctant to tell Craig that she's a mermaid. Piper and Phoebe are reluctant to help, much to Paige's frustration. Paige pulls Piper and Phoebe into the kitchen. Phoebe is concerned about her court date, but Paige thinks she and Piper can handle the Sea Hag. However, Piper is reluctant to face her, largely because she kills in the same manner that her mother was killed by the Water Demon. Leo notices Piper is a bit shaken.
Necron's minion appears in the Sea Hag's lair, followed by Necron himself. The Sea Hag too is up against a deadline--six months earlier, she talked Necron out of killing her by promising to give him a mermaid's immortality. The Sea Hag promises an even more powerful storm to find Mylie. Necron kills his minion instead and tells the Sea Hag she'll be next unless she can get Mylie.
Paige tracks down Craig at the airport and tries to talk him into talking to Mylie. He initially refuses, but changes his mind after Paige says Mylie's life is in danger.
Phoebe and Darryl are before a judge. Phoebe claims to have expended considerable effort to find Cole, and Darryl says that he believes Cole left the country. The judge grants the divorce, but just as Phoebe is about to sign the decree, Cole walks in. Phoebe is very angry at Cole, and Darryl is upset that Cole made him look bad in front of the judge. Cole insists that he's good, but Phoebe wants him out of her life. Darryl wants to chew Cole out, but Cole waves his hand and turns him into a water cooler, further angering Phoebe. Cole says that he's acquired a raft of powers so he can redeem himself. He vows to not give up on Phoebe. Enraged, Phoebe grabs a letter opener and points it at Cole. Cole tries to take it out of her hand, but Phoebe slashes him. A few drops of blood fall on some papers. Cole's wound magically heals, and the blood on the paper starts to bubble like acid. Horrified, Phoebe runs away. Cole turns Darryl back to normal.
Piper has looked up the Sea Hag in the Book of Shadows. She has control over the water, and Piper thinks that may explain the heavy rainstorms. Mylie mentions that the Sea Hag can even stir up tidal waves, unnerving Piper. Leo senses something is wrong, and tells Piper to try to keep Mylie calm while he checks with the Elders. Craig and Paige walk in, but Craig still refuses to take the situation seriously. Mylie wets her feet and turns into full mermaid form. A horrified Craig leaves and the Sea Hag tracks Mylie to the manor. Piper tries to freeze her, but it has no effect. The Sea Hag throws a water ball at Paige and traps her in a tower of water. She throws another water ball at Piper, but Piper ducks behind the couch. Mylie calls for help, but Piper is frozen in fright. Piper comes out behind the couch to find Paige coughing and Mylie gone.
Phoebe and Paige are shocked that Piper lost an innocent, but Leo believes Piper's maternal instincts are kicking in. Piper found a spell to vanquish her. Unfortunately, the Elders told Leo that they need a mermaid to track her down. Paige realizes they have to find Mylie before she gives up hope--and her immortality. Leo orbs to the airport to find Craig. Phoebe and Paige talk Piper into resting while they find the Sea Hag.
Mylie is distraught over Craig. The Sea Hag sees this, and offers her the auger shell again.
Darryl is pinned down by a machine-gunner in a diner. The criminal sneaks up behind Darryl to ambush him and he shoots. Suddenly, the bullets slow to a stop. Cole shimmers in behind him and offers to help him get the collar as part of his effort to prove he's good. Darryl tells Cole it's a waste of time; Phoebe doesn't want him.
Paige has written a spell to track down the Sea Hag. Her cell phone is ringing off the hook; it's Cowan wondering where she is. Paige starts to wonder if being a social worker is really her destiny. She notices Phoebe is somewhat moody. Phoebe tells Paige that Cole is back. She shows Paige the letter opener she slashed Cole with; his blood caused the tip to eat away. Phoebe wants to run and hide; she feels Cole has sucked all the fight out of her. Paige offers to find a way to vanquish Cole for good. Phoebe and Paige aren't sure if Piper is up to hunting down the Sea Hag. Just then, Piper walks in; she wants to find the Sea Hag for Mylie's sake. The sisters cast the spell, but nothing happens at first. Suddenly, Phoebe doubles over and turns into a mermaid. Phoebe reluctantly enters the ocean just as Leo orbs in. Phoebe pops up; the call of the sea is every bit as strong as Mylie said it was. She senses a stench in the water and tracks it to Mylie and the Sea Hag just as Mylie is placing the shell on her heart. Phoebe calls for Leo. He, Piper and Paige start to orb away, but at the last second Piper panics and lets go. Paige orbs the shell away from Mylie. The Sea Hag encases Paige in seaweed. Leo grabs a sword and swings it at the Sea Hag. He misses, and the Sea Hag traps him in a tower of water. Phoebe throws the shell at the Sea Hag's heart, vanquishing her.
Mylie is near death, and Leo can't heal a mermaid. Thinking Craig is the only one who can save her, Paige has Leo orb Craig off of his plane. Leo convinces Craig to tell Mylie that he loves her. Mylie is now fully human, and Leo can heal her. Leo senses Piper's panic and wants to go back to her. However, Phoebe is still a mermaid, and refuses to return to land.