As Jessie enjoys herself in a boutique store, the rest of Team Rocket dread having to carry her stuff back home. Just then, Mareanie spots a Corsola horn necklace on a store stand, though James explains they cannot afford it at the moment. As the group leaves the store, Jessie exclaims that shopping is her goal in life, though Meowth corrects her, saying that they should be trying to capture rare Pokémon in Alola. Suddenly, they see a wild pink Frillish in the sea, which Meowth states is female. James attempts to capture her, but Mareanie attacks it with Spike Cannon, which scares it away. James scolds Mareanie for attacking Frillish, making her cry. Meowth explains that Mareanie could not let another female have James, with Jessie adding he can't even forgive a lady for being jealous. James apologizes to Mareanie, who hugs his face and poisons him in her affection. When James collapses, they encounter another wild Mareanie, which is male and of a different color.
Meowth believes that the male Mareanie is James's Mareanie's first love. He and Jessie see the physical resemblance between the male Mareanie and James, and add that they are both unreliable and indecisive, much to James' annoyance. The male Mareanie hops towards James's Mareanie, only to receive a slap. When Jessie asks for Meowth to translate the pair's conversation, Meowth reveals that the female Mareanie was searching for love, and one day, she was about to be attacked by a Tentacruel when the male Mareanie rescued her. James's Mareanie then developed a crush on the male. After the male inquired, the female Mareanie admitted she was searching the beach for a Corsola horn; an item most of her friends had. The male Mareanie promised to find her one. However, days later, the Mareanie saw the male Mareanie giving a Corsola horn to another Mareanie. Out of jealousy and heartbreak, Mareanie never saw the male Mareanie again, until today.
Meowth overhears that the male Mareanie was just returning a Corsola horn to the other Mareanie after she dropped it. He then presents James's Mareanie with a Corsola horn, though she declines and goes to James, grasping his leg in affection. The male Mareanie becomes jealous and attacks James with Spike Cannon. Suddenly, the male Mareanie evolves, and James's book confirms that it is called a Toxapex. Toxapex surrounds James's head and poisons him, but his new purple complexion makes him even more attractive to Mareanie. Mareanie is also angered by Toxapex's actions and charges forward to attack, though Toxapex flees. Jessie and Meowth note that James has been poisoned badly, but just as they go to search for a doctor, Bewear appears from under the sand and takes them back to its den.
At Bewear's den, Jessie and Meowth are concerned that James has still not recovered. Mareanie fetches some water for her Trainer, but upon her return, she overhears that James has decided to let her go. James explains that Toxapex brought her a Corsola horn and evolved in her honor. As a result, he believes that Toxapex cares for Mareanie much more than he does and would make her happier. Meowth and Jessie chide him for this decision, though they offer James their support in his time of ill health. Mareanie runs off, spilling the water bucket in the process.
That night, Ash, his Pokémon, Professor Burnet, and Professor Kukui are walking along the beach. Ash then notices a sad Mareanie, sitting on a piece of driftwood, ahead and invites it back to Kukui's house. Once there, she sits alone, not wanting to be disturbed. Burnet's Munchlax offers her a Pecha Berry to make her feel better, but Mareanie refuses. They believe that it is Team Rocket's Mareanie, and plan on returning her to them. Burnet finishes preparing a special meal for Mareanie, but she refuses to eat. Munchlax is interested, but Burnet reminds it that the food is for someone else. Ash laughs, noting that Munchlax ate already. Mareanie looks over at Ash and is reminded of James for a split-second.
Overnight, Mareanie drifts off to sleep and dreams of the time she spent with James. At morning, Mareanie wakes up and decides to take a walk on the beach. While walking, she is ambushed by a lurking Tentacruel, but James arrives and kicks the Tentacruel away. Mareanie is overjoyed and jumps onto her Trainer's torso. James hugs her and apologizes for the delay in finding her. Toxapex arrives on the scene, and James promptly puts Mareanie down and pushes her forward. He explains that Mareanie will be happier with Toxapex and pushes her further, though she refuses to budge. James eventually shoves her in frustration, causing her to fall into the sand. She picks herself up, only to see James with his back turned and walking away. Jessie questions his decision, whilst Meowth remains silent with tears in his eyes. James affirms his decision to release Mareanie, as he looks forward to poison-free days and uninterrupted Pokemon catching. Toxapex pipes up and strikes James with its Spike Cannon. Meowth translates its outrage, and clarifies that it doesn't want to see Mareanie cry again and thus demands a battle. James sees Mareanie's tears and agrees to fight Toxapex in honor of Mareanie.
The battle ensues, with Toxapex launching a Spike Cannon, which James avoids. The rest cheer for James, who tries to attack, but Toxapex dodges it and leaves James's head in the sand. Just as James gets his head out, Toxapex latches onto him and poisons him. Much to everyone's surprise, James resists the poison because of his passionate anger, claiming this is his evolution. The sudden immunity changes that battle into a fistfight, which Mareanie enjoys. Out of exhaustion, the two concede and end the match in a draw; Meowth proclaims that the two enemies are now friends. Mareanie thanks Toxapex for challenging her Trainer, until a wild Tentacruel attacks them. James intervenes, this time grasping Tentacruel's two tentacles and swinging it away. Toxapex gives a Corsola horn to James as a token of its gratitude, but the gesture only infuriates Mareanie. After chasing off Toxapex, Mareanie embraces her Trainer and poisons him again.
Ash wakes up to see Professor Burnet standing by the bare couch and asks her where Mareanie went. She subtly looks out of the window and assures Ash that Mareanie is just fine.