Phoebe is enjoying her new life as a mermaid—an unexpected side effect of the spell to find the Sea Hag. Although Leo can track Phoebe, he can't convince her to come home. Phoebe's career as an advice columnist has made her a minor celebrity, but her personal life is a shambles: Cole is back from the Wasteland, and all Phoebe wants to do is run (or more precisely, swim).
Necron, a skeletal being trapped between life and death, returns to the Sea Hag's lair in search of the immortality she promised him. But all he finds is a pile of dust—what's left of the Sea Hag since the Charmed Ones vanquished her. Necron feeds on the Sea Hag's life essence, which was trapped in an augur shell. But he still hungers for immortality—the immortality he can only get from a mermaid.
Piper, in her first trimester of pregnancy, struggles with her growing fear. She's worried that like her own mother, she'll be killed by a demon before her child grows up. She realizes that to save Phoebe, she must remove her fear, so she casts a spell:
Full of fear
My panic turns manic
Till I can't hear
In need of reprieve
So that I can breath
Remove my fear
Please make it leave
Cole approaches Paige at her new job as a social worker. He's back with the law firm, and he wants to help Paige with her cases to help make up for his evil past. Paige tells him to stay away from her—and from Phoebe.
"She doesn't love you," Paige says. "She thinks you're an evil freak with superpowers from hell and battery acid for blood…She wishes you'd stayed dead."
"She told you this?"
"Told me?" Paige says. "She didn't have to tell me—she's a mermaid. Her body erupted in scales. She had to turn into a fish to get away from you. Do you get it now?"
Phoebe is captured by fishermen who consider killing her and selling her body to the tabloids. Necron appears and turns the fishermen to dust. Phoebe calls for Leo, and he orbs her home, saving her from Necron's attack. Necron sends two crows to locate her.
Piper finds Necron in the Book of Shadows. She mixes a powerful potion that nearly destroys the attic. Paige and Leo wonder about the sudden change in her, and they find the fearless spell she cast.
Darryl goes to Cole's apartment, wondering if Cole might have had anything to do with the fishermen turning to dust. Cole swears he's changed, and he wants Phoebe back. Darryl convinces him that the best thing he can do for Phoebe is to stay out of her life.
While in the bathtub, covered with bubbles, Phoebe conducts an interview with Nancy O'Dell. Nancy says, "Your advice to lovers, it's so real, so heartfelt, especially lately. Is there a lucky guy in your life that you get your inspiration from?"
Paige realizes she was wrong—Phoebe's still in love with Cole, and it's her love that she's running from.
After Nancy leaves, Piper spots two crows on a tree branch staring through the window. A moment later, Necron appears. Paige orbs Phoebe to the attic, and Necron takes Piper back to the Sea Hag's lair. Necron chains Piper to a metal grate. He tortures her to convince her to call for her Whitelighter; but under the influence of the fearless spell, she merely mocks him.
Looking through the Book, Paige discovers that the last two witches who vanquished skeletal beings were vaporized in the process. She believes that using a potion against Necron would be a mistake—their best hope is a power-of-three spell.
Leo orbs Phoebe to the beach, but Paige lets go of Leo's hand and orbs to Cole's apartment instead. Cole is planning to leave town, but she tells him she was mistaken about Phoebe's feelings for him. To prove it, she casts a spell so he can feel what's in Phoebe's heart. Cole agrees to help Phoebe confront her feelings so she can turn human again.
At the beach, Leo can sense that Necron is hurting Piper. Phoebe swims to the Sea Hag's lair to buy them some time until Paige gets there. Necron zaps the auger shell onto Phoebe's chest to suck out her immortality. Paige orbs in with Leo and calls for the shell. Piper and Paige grab Phoebe's hand and recite the spell, vanquishing Necron. But the explosion knocks Piper into the water. Her foot is still chained to the grate, which drags her down, trapping her under water.
Piper frantically tries to free herself. She's about to give up when Patty appears to her and says, "Piper, I know why you've been so afraid. You don't want to leave your baby the way I left you. But spells won't make that fear go away, only faith will. Have faith that your destiny is different than mine. Take my hand."
Piper reaches for her mother, and her foot slips free of the chain. Phoebe pulls Piper to the surface. But before Leo can heal Piper's wounds, they heal magically—apparently the work of the baby Piper is carrying.
Cole uses his powers to summon Phoebe to the beach where he's waiting for her. "You thought you could escape into this big ocean," he says, "that the waves would wash away your pain. But they won't. Not until you admit what drove you here in the first place."
Realizing the truth, Phoebe turns human again. "I do love you," she says, "and I always will. But it doesn't change anything…It's over between us."
Life turns back to normal—or a new version of normal. Phoebe files for divorce. Paige quits her job, realizing that she can't be a good social worker and still be a good witch. Piper places a photo of herself and Patty into the baby book she's putting together for the child she's carrying. Patty magically writes, "Grandma and Mommy" on the page. Piper smiles.