The story follows Sora, Kairi, Donald Duck, and Goofy as they travel to distant, previously unvisited worlds to combat the ever-spreading Heartless. However, Yen Sid and King Mickey also discover that an ancient evil long thought defeated has returned, and threatens all worlds.
Kingdom Hearts: Forgotten Worlds contains examples of the following tropes:
- Ancient Evil: Whatever is spreading the Heartless' reach is implied to be the same evil Yen Sid and King Mickey are worried about.
- Artifact of Power: The artifact that the Master seeks will supposedly allow him to carry out his evil plans, as it "demands death".
- Crossover: As with the canon Kingdom Hearts games, Disney properties and Final Fantasy characters meet in the same story. This story takes it a step further by including Disney-themed worlds and characters that did not appear in the canon games.
- Hidden Villain: The ancient evil simply known as "The Master" commands a New Council of Villains and seeks an object of power that will allow his evil schemes to unfold.
- Jerk with a Heart of Gold:
- Lightning is a moody, distant young woman who shuns and belittles Sora and co. for bothering her and offering to fight alongside her. However, after they prove themselves by helping her defeat the Great Defiler, she softens her attitude towards them and offers to aid them on their quest, admitting that she had something bad happen to her to cause her poor attitude.
- Emperor Kuzco starts off selfish and rude, but over time it is revealed that he is kinder deep down than he cares to admit. By the end of the chapter, he lets his humility and kindness shine through, thanking everyone and even giving Sora a gift.
- Legion of Doom: After the fall of the first Council of Villains, the Master forms the New Council of Villains consisting of himself, Maleficent, Pete, Bill Cipher, and three mysterious new members.
- Mix-and-Match Weapon: Lightning's weapon of choice is her signature gunblade.
- Sealed Evil in a Can: Lightning reveals that Dorgann Klauser was hiding a steel box, which had a powerful Light spell cast on it to keep something Dark inside. Whatever it was escaped after Klauser passed away, and is likely what has been spreading the Heartless and the Darkness to new worlds.