The Hardest Thing

"The Hardest Thing" is the fifty-eighth and final episode of the Disney Channel animated series, Amphibia; serving as the series finale.

Plot
Following their return to Amphibia, the Core, still trapped within the helmet, flies up into space and attaches itself to a pod located up on the moon. It then proceeds to activate it and turns on hidden thrusters; causing it to fly towards Amphibia. As everyone looks up, Andrias reveals that the Core made a base up on the moon as a final gambit to destroy all life as it is afraid of being irrelevant. Mother Olm arrives to inform everyone about the prophecy; revealing that it is not a prediction, but rather a request that the three girls, Anne, Sasha and Marcy, help save Amphibia from destruction. However, this would result in the total depletion of the gem energy and possibly trap them in Amphibia forever. The girls nevertheless agree for one final adventure.

Before leaving, Mother Olm tells Anne that there may still not be enough energy to defeat the Core and that she would need to use a "secret spell" in order to defeat it. This spell can only be wielded by one and could result in their death. She tells her that she "have only but to ask" when it comes time to use it. Anne runs into the castle and is surprised by the appearance of Valeriana who has all three gems in the music box. Using her abilities, the gem powers are charged into the girls who are now all empowered enough to fly and exert energy. They all fly into space where they proceed to destroy numerous robots and monsters sent out by the Core and try to push back the moon.

Andrias sends his drones up into space as a shocked Grime watches. However, to his surprise, Andrias orders the drones to help push the moon back. He takes his crown and tells the Core that he is finally doing the right thing by standing up to him and crushes his crown, taking his connection to the Core in the process. Despite this, the Core is still too powerful and Sasha and Marcy's powers begin to wane due to inexperience. Frustrated, Anne convinces them to let her absorb all their powers and she solemnly sends them back to Amphibia. After being informed about the situation from Sasha and Marcy upon their return, Sprig has Frobo take him to Anne, refusing to lose her. As the Core berates Anne's use of "its greatest treasures" against them, she states that the stones never were theirs. When she asked the stones if they would help her save the world, they shatter and transfer their power to her. Now all powerful, Anne sees Sprig beg her not to sacrifice herself as she changed his life, only to tell him that he changed hers as well. After bidding her friend farewell, she uses all her energy to blast at the moon before flying through it; destroying it and the Core. As the others celebrate, Sprig and Frobo return a pale singed Anne to the ground. As everyone cries over her, Anne assures her decision as she deteriorates into a pile of leaves that drift away.

Anne awakens in a limbo-like dimension resembling a tiny house and garden floating in space. As she enters the house, she speaks to the Guardian of the Stones via an old fashioned computer. When Anne mocks its dated appearance, it transforms into Domino and proceeds to converse mentally with her. The Guardian reveals that she is a "backup" and that she is not dead or alive. It goes on to reveal that it has watched over the universe for a "very long time" and wants Anne to be its replacement as she is the only person who has used the stones for selfless purposes. Anne immediately turns it down after admitting that she is not perfect and that she would need years to become wiser. Intrigued by this notion, the Guardian decides to return her to life and will wait "78 years" for her to accept, much to Anne's confusion.

After being revived, Anne awakens back in Amphibia and reunites with her friends who are overjoyed to see her alive. She also discovers that the Guardian left her with three tiny gem shards. The next day everyone says goodbye with Marcy stating that the gems are only enough for the trip back home. She apologizes to the newts for technically being the cause of all their troubles, but Olivia and Yunan thank her for changing their lives. A repentant Andrias, quietly tells her to take care. Sasha and Grime have a comically tearful goodbye with the former telling the latter to find Percy and Braddock and tell them that she will miss them too.

Anne finally says goodbye to the Plantars, especially Sprig whom she leaves her cellphone to help with future adventures. As Valeriana opens the portal back to Earth, Anne has one final hug with Sprig before all girls leave for good. The portal closes, followed by the gems getting drained and the music box finally crumbling to pieces. As the Amphibians all agree to rebuild their world for the better, Polly suddenly sprouts a strand of hair; exciting her.

Roughly nine months later, Andrias now lives a peaceful life gardening with his robots. As One-Eyed Wally leaves Stumpy's he bumps into Polly, who has now become a teenaged frog. Polly runs into town passed Maddie who started her own pan and potions business with Mrs. Croaker as a customer and Loggle who has reverted to his smaller self after taking a "cheat day". Grime, Olivia, and Yunan arrive on Joe Sparrow where they are surprised to find that Toadie won the recent mayoral election with Toadstool having supposedly helped him out. Olivia expresses to Yunan that she wouldn’t mind settling down in Wartwood someday, although Yunan seems eager to talk about her former life as a general. Joe Sparrow and Bessie somehow mated and had several hybrid offsprings.

Polly arrives home where Frobo continues to garden for Hop Pop's California Avocados. Hop Pop himself has settled with Sylvia Sundew who tells Polly that Sprig is inside the house. Sprig has started charting the things he has learned into a journal and is invited by Polly to the new town's unveiling of a statue of Anne Boonchuy. Sprig is suddenly tackled by Ivy who tells him that Grime has discovered a new continent that has not been set foot on by frogs, toads or newts and ask if he would like to come along. After having one emotional look at Anne's statue, Sprig agrees and heads off with Ivy for a new adventure.

Back on earth, ten years have passed since the publicly known "frog-vasion". Sasha picks a still clumsy Marcy up at the airport to visit Anne for her birthday. Marcy has become a webcomic author while Sasha has become a psychologist for children. She reveals that she and Anne stopped spending time together once they entered high school, though they still kept in touch. Anne is revealed to have become a herpetologist and now works at the Aquarium of the Pacific. She teaches a young class of children about the Pink South American Tree Frog (the species that Sprig is and whom she named it after) and is shown to be very happy with her life. After putting the frog back, Anne is greeted by Marcy and Sasha and the three friends leave to hang out, ending the series.