Kingdom Hearts III begins with a chess match between a Young Xehanort and Eraqus, with the two of them representing darkness and light respectively. As they play, Xehanort ponders over the Keyblade War and the Lost Masters who started it. He quotes the prophecy which says "On that land shall darkness prevail and light expire." Eraqus expresses his doubts that anything is inevitable, but Xehanort looks into the Gazing Eye of the No Name Keyblade, which would one day become his.
In the present day, Sora once again undergoes the Dive to the Heart, this time with him being surrounded by memories of his past adventures. He is given two sets of three choices: the first of which dictates his initial health and magic, and the second of which dictates the order in which he learns his abilitiesnote . He is soon swept up by a tidal wave into a neverending sky and sea, and ends up fighting a semi-transparent Darkside...
Following a retread of the ending of 0.2, Sora, Donald, and Goofy set off to visit Hercules in order for Sora to regain most of his lost power, and thus begins the plot of… er, Kingdom Hearts II.9.
As they open a gate to Olympus, Hades, having failed all of his past plots, returns to his original scheme that he planned eighteen years ago: once the planets were in alignment, he could release the Titans from their prison and set them on Olympus. When Sora and friends arrive, they end up on Mount Olympus itself, not in the coliseum. Soon they encounter Hades, who summons the Titans and uses them to blow the trio down into the city of Thebes. Afterwards, Maleficent and Pete show up, much to his annoyance. Since all his previous attempts at cooperating with them failed, there is no love lost between them and Hades. Maleficent asks him if there is a black box in the world. Hades recalls a box that Zeus hid on earth, so he directs the two of them to it.
At Thebes, Sora and friends are thrown right to where Herc is. He explains that Hades is trying to take over the cosmos with the Titans. Sora tries to ask him how he got his strength back, but Herc can't really explain it properly. The heroes are interrupted when some Heartless show up, so they fend them off. Meg and Phil show up on Pegasus. After a short reunion, Herc sends them away to safety. The group spots a young girl in danger. Herc has the other three get on a statue, which he throws in her direction. They save the girl before being approached by Maleficent and Pete. Pete notices that Sora is weaker, but Maleficent pays no mind to him. Pete accidentally lets it slip that they're looking for the box before the two villains leave.
As the three friends venture further through Thebes, they get stuck when they encounter a big fire, but Goofy comes up with an idea: they can use his shield as a sled of sorts to get across. On the other side, they see a woman on top of a pillar surrounded by Heartless. After getting rid of them, the woman climbs down and thanks them. Later, they find a burning building that Herc is struggling to hold up. They deal with some Heartless and get the people inside the building outside, but the building collapses on Herc. He emerges safely, but then Xigbar appears to confront them. He mocks Herc, saying that risking your own life for others isn't admirable as it will just get yourself hurt. Herc tries to say that he saved Meg thanks to his courage, but Xigbar points out that he only became a "true hero" because of his divine parentage, meaning such heroic acts of sacrifice often don't work out well for normal people. He urges Sora to keep going so he can find the hearts connected to him before disappearing.
The heroes soon reach Mount Olympus, where Herc tells them that his father is Zeus, king of the gods. This comes as a surprise to Sora and friends, who had never heard this before. Herc goes on ahead to see if his family is safe, leaving the other three on their own. Near the top of the mountain, they encounter the Rock Titan. After defeating it, they reach the Realm of the Gods, where more Heartless appear. The trio defeats them and arrives at the top, where Hades has imprisoned Zeus and the other gods. Herc frees the chained gods while Hades sends the remaining three Titans after them. After a battle, they seemingly win, but the Titans are still alive and well; what's more, the Rock Titan returns to join them. Herc frees Zeus from his prison, and he easily injures them with his lightning bolts. Herc grabs the Tornado Titan, uses it to suck the other three up, and flings them out into space, where they explode.
His plan foiled, an enraged Hades almost picks a fight with Sora and friends, but when he realizes that he's outnumbered, he retreats. Herc decides to go back to the realm of mortals to stay with Meg, saying his life would be empty without her. Sora comes to the conclusion that he has to get his strength back on his own, which he realizes was the answer he was looking for all along. Meanwhile, Pete is digging for the black box on Maleficent's orders. He finds a box, but it turns out to be Pandora's Box, not the one they are looking for. As the two of them leave, Xigbar watches over them. He begins to say "May your heart be your guiding key," but he is cut off mid-sentence. With that, both the heroes and villains move on to another world...
Tropes:
- Aborted Arc: Sora goes to Olympus to seek out Hercules in order to regain the power of waking. Turns out Herc has no idea how he regained his strength, so Sora decides to find it his own way.
- Art Evolution: After spending the last few games without it, Hades finally regains the smoke at the bottom of his robe he had in his movie.
- Exponential Plot Delay: Just before Sora and company get to Olympus, we get a title card of... Kingdom Hearts 2.9, as if this was yet another interquel.
- Serial Escalation: The Rock Titan fight, which this time has Sora dodge boulders it flings and scale the beast in order to hit it. For that matter, he gets to fight three Titans at once, too.
- The Unfought: Hades. Sora still has to "feel the heat" in the form of the Lava Titan, though.
- Wham Line: Xigbar saying, "May your heart be your guiding key," the first hint that he's really Luxu.
