"Happy Couple Again" is an episode about Finn and Flame Princess after they got back together in "Fire & Light" and the problems that they must solve in the Fire Kingdom.
Finn and Flame Princess were playing together in front of the Tree Fort, happy to be together again. Just about when Flame Princess was about to do another trick, she stopped and recalled back to the situation when she battled Solar Princess. She turned to Finn and told him that she wanted to check on Cinnamon Bun if he is alright in case Solar Princess is evil. Finn wanted to come with her, but she denied and said that it is too dangerous.
In the Fire Kingdom, the princess easily defeated the guards and went in. She saw Flame King and Solar Princess sitting together, with Cinnamon Bun trapped in her old lantern. For some reason, Solar Princess was looking a bit dizzy and unwell. Flame Princess thought of if her father and the Solar Princess teamed up. Suddenly, Flame King revealed the whole plan in pride: First, he tricked FP that her mother is evil, and after she left, he kidnapped Solar Princess and brainwashed her into thinking that he is her boss. He told Solar Princess to lure Cinnamon Bun in, which she acceded. He then trapped him in the lantern.
Flame Princess, who was hiding behind a wall near the throne, was confused on why Flame King wanted to trap Cinnamon Bun. Flame King continued with his plan: Later, he would brainwash CB as well and tell him to start a war on two kingdoms. He can tell the Candy Kingdom how bad the Slime Kingdom is and start a war. In the middle of their war, the Fire Kingdom can come and burn the two kingdoms out and take over their lands. And so, CB can continue to do this and the Fire a Kingdom can take over the whole land of Ooo. Flame Princess became outraged and formed a sword out of her hand. She charged in before her father could lower the lantern. She opened the lantern and lifted CB out, who is covered in Flame Shield.
The Flame King and Solar Princess, as expected, attacked the princess. While FP was trying to hold both of them, Cinnamon Bun tripped over a rock nearby and fell. The rock flew over and hit Flame King, knocking him out while the two princesses continued their battle. Flame Princess pleaded Solar Princess to come back to her senses, but she did not seemed to hear. Flame Princess had no choice but decided to defeat her first. She pointed to the Flame King, who was knocked out, to distract Solar Princess. When the plan worked, Flame Princess leaped and kicked Solar Princess twice in mid-air, causing her to fall.
Just then, Finn and Jake came. They asked if she was alright. Flame Princess picked the crown from her father and put it on her head, once again becoming the ruler of the kingdom. She turned to the boys and explained that the Fire Kingdom cannot survive without her. She picked Solar Princess up and gave it to Finn. She told him to transport her to Princess Bubblegum to see if she is still brainwashed. The boys nodded. Then, Flame Princess exited the scene to check if the citizens are alright. After the princess left, Finn asked if she had broken up with him again. Jake shrugged and the episode ends.