WARNING: The following contains spoilers for One Piece: Ace's Story, Vol. 1, by Eiichiro Oda, Sho Hamazaki and Stephen Paul, available in English now from Viz Media.
Like ki in Dragon Ball and Quirks in My Hero Academia, Devil Fruits are the strange source of superpowers in One Piece. Consuming one of these sinisterly-named fruits will render you useless in the water but extremely useful in battle. One of the most famous inheritors of a Devil Fruit power is Portgas D. Ace, son of Gol D. Roger and adopted brother of Monkey D. Luffy. While he was alive, Ace possessed the power of the Mera Mera no Mi, better known in English as the Flame-Flame Fruit. However, while fans of Eiichiro Oda's manga/anime series knew of Ace's fiery powers, they've never really known the full story of how he gained them -- until now, thanks to the first volume of the spinoff light novel, One Piece: Ace's Story.
At the start of the prequel story, Ace is shipwrecked on a deserted island alongside an unnamed adventurer who later takes on the name of Masked Deuce; becoming Ace's first mate in his Spade Pirate crew. While the two are fundamentally different -- even clashing over Ace's relation to the legendary outlaw Gol D. Roger -- the two end of working together in order to build a raft to escape.
Naturally, they fail far more than they succeed, with their vessels constantly falling apart before getting all that far out to sea. In the meantime, they also search for food in order to survive, leading to Ace finding the Flame-Flame Fruit entirely by accident. He takes a bite of it first before offering it to Deuce, who doesn't particularly like how the fruit tastes. It's probably a good thing Ace offered the fruit to Deuce, though, because his island-mate was considering killing Ace and stealing the fruit for himself before Ace's act of generosity. Additionally, Devil Fruit powers only grant the person who takes the first bite any powers, meaning that, had Deuce successfully betrayed Ace, he would've become the inheritor of the Flame-Flame Fruit.
Ace quickly realizes that the fruit is a Devil Fruit after spontaneously sprouting flames from all over his body. This complicates his situation since those who eat Devil Fruits can't swim. However, as he knows Luffy ate the Gum-Gum Fruit, he understands the difficulties that come with it without having to discover this downside for himself, as well as the potential benefits.
It's thanks to his new fire powers that he's able to figure out a way to effectively construct a raft in thee end and, with Deuce's help, build what became known as the Striker. Because of this accidental mishap -- and some pre-meditated murder along the way -- Ace is able to take his first step toward becoming the pirate everyone knows and loves.
