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Mickey's Letters refer to the three letters King Mickey has written over the course of the Kingdom Hearts series. The first letter was left by King Mickey on his throne for Donald Duck, before he left to seek out the Keyblade. The second letter was carried by Pluto as he led Sora, Donald, and Goofy from the End of the World to Castle Oblivion, and the third letter was sent by Mickey to Sora and Riku to explain the situation behind Terra, Ventus, and Aqua and summon them for a Mark of Mastery exam. Each letter was sealed with a green Mickey Head.
In the manga, the letter is worded differently, replacing Mickey's informal dialect.
When Donald enters the Disney Castle Throne Room to give Mickey his morning greeting, he finds only Pluto, with the first letter. He rushes to show it to Goofy, while trying unsuccessfully to keep it hidden from Minnie and Daisy. With the help of Chip and Dale, Donald and Goofy are sent to Traverse Town to talk to Leon, where they serendipitously stumble upon the Key-bearer, Sora. Per Mickey's orders, they join up with him.
When Riku takes the Keyblade from Sora, Donald and Goofy follow him, because Mickey had specifically said to stick with the person who had the key. However, when Riku attacks Sora, they defend him out of friendship, and Sora regains the Keyblade.
Shortly after arriving at Traverse Town with Donald and Goofy, Pluto senses King Mickey's presence and runs off. After meeting up with the King, he receives a second letter that he travels across the Realm of Light with, visiting many worlds that Sora had not seen in his first journey.
Some time after Sora, Donald, and Goofy defeat Ansem, Seeker of Darkness, and end up in the meadows surrounding the restored Castle Oblivion, they come across Pluto with the second letter, and chase after him until they come to the castle. The contents of this letter are never revealed.
After meeting the Data-Naminé within the Journal and discovering the truth behind the secret message, she requests that the King explain the situation to Sora, so King Mickey sends Sora, Riku, and Kairi a letter in a bottle. The letter explains that all of the strange events in their journey were connected, and that the trio must go on one more journey to solve the remaining troubles of those connected to his heart.
The letter in a bottle quickly arrives on the shores of Destiny Islands, and after Kairi rushes to give it to Sora and Riku, the trio reads it together. Shortly afterward, Sora is sitting on the paopu tree when Riku walks up to him and asks if he has decided yet. Sora, holding the letter, replies that he has. Kairi walks up, and Sora explains to her that there are still sad people, so he must once again leave to put right everything that is connected to him. Kairi smiles, gives him back her Wayfinder, and tells him to hurry back.
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