The show begins right after the episode "Incendium" left off with the Flame Princess telling Finn to never mess with her again, slapping him in the face, then leaving. Jake then explains that he was trying to help Finn get over his Princess Bubblegum sad times" but then explained that she was evil. Finn became extremely mad at Jake for saying she was evil and claimed he "peeped beyond her soul" and that she was, in fact, not evil, just passionate. Finn then convinces Jake to help him track her down. Jake begins to walk over the forest with Finn and try and convince him to date another princess and even suggests Wildberry Princess as an option. Suddenly a Brown Bird flies above that was set on fire by the Flame Princess and directs Finn and Jake to the location of the Flame Princess. Jake then shrinks down and he and Finn start to spy on Flame Princess. They view Flame Princess curiously playing with and burning a few flowers. Finn was so attracted to her by just looking at her that he admitted that she looks like a steam of a puppy's nose searching for ham in the snow. While spying on her, both of them continue to argue about whether or not she is evil. The conversation is stopped abruptly when Flame Princess hurts herself by putting her hand in a pond.
Finn blows his cover just to see if Flame Princess is alright and explains how he feels which causes her to glow brighter in excitement. Finn got scared of the fire that Flame Princess produced and he puts it out, and it is revealed that destroying the fire that Flame Princess creates actually causes her physical harm. She gets scared and runs away and Finn chases her and Jake follows into an open field where Flame Princess ambushes them.
The attack does not last long and Finn confuses Flame Princess by trying to compliment her while she is trying to kill them. He explains he wants to be with her which causes her to blush but she resists and then explains that all Finn has done is hurt her and that she thinks that Finn is trying to impede her, and to take revenge she threatens to transform the Goblin Kingdom into her very own personal fire kingdom. She starts putting the entire field ablaze. Finn tries to convince her not to do it but he and Jake were restrained by a wall of fire. Jake told Finn to fight her but he was still into her. Jake asked him whether being with her or being a hero and saving poor innocent Goblin folks was more important. Finn had no choice but to be a hero. Then Jake informs Finn of his plan to build fire proof suits. Finn and Jake return to their tree house and go into the garage in an attempt to find some raw materials to build their suits. Instead, they find NEPTR who was playing hide and seek and claims he was hidden for over 15 months, 4 days, and 9 hours. NEPTR then helps build fire-proof suits for Finn and Jake to wear. The suits are huge metal robot-like shells equipped with rockets and foam blasters and NEPTR as a helmet of Finn's suit.
They all fly to the Goblin Kingdom where they try and reason with Flame Princess, but she continues to rampage and tries to attack Finn and Jake, but it doesn't work because the suits are fireproof. Flame yet again shoots another fireball at Finn, but it was deflected and shot at a goblin's cart (where a goblin said that he was born). Jake then attempts to put out some of the fires and injures FP in the process. Then NEPTR attempts to put out FP by blasting her with the foam blaster, but Finn manages to stop NEPTR from directly hitting Flame Princess and instead he hits Jake and a few of FP's flames. Flame Princess then gets extremely angry and proclaims "that is the last time you hurt me" and grows into her beast form and continues to rampage through the Goblin Kingdom. A tear from FP's right eye is revealed that she was harmed both physically and emotionally.
Finn then literally rips himself out of his own suit (never again wanting to see her getting hurt) and stands on top of it and shouts to Flame Princess. She ignores him, and Finn says he failed everyone. NEPTR says "Creator hasn't failed NEPTR." Finn begins to cry and some of his tears fall on the flames of Flame Princess and not only hurt her, but catch her attention. Flame Princess watches Finn cry for a little then stops all her flames from destroying the city. She says she thinks she understands now, and that Finn is a Water Elemental because he creates water (tears). Finn tries to explain that he isn't but Flame Princess has trouble understanding the concept of his species and says even if they liked each other, they would only hurt each other. Finn says "No, I can take it, I mean, can we try?" Then she replies "You would defy nature for me?" And Finn says "Yeah, whatevs," then the Princess hugs Finn, and he gets burned and begins to tear up again. Flame Princess says goodbye and leaves in a trail of flames towards what seems to either be the desert or just a simple sunrise. Jake then goes over to Finn and tells him that all of the goblins were "barbequed," but was just messing with him, and then reported that he saved all the goblins and asked how things went with Flame Princess. Finn tells Jake "She hugged me, then left." Jake replies "Whoa, how did it feel?" and Finn ends the episode with the line "It hurt," smiling.