Second year high school student Koguma doesn't lead a very interesting life. She has no parents, and no friends nor hobbies to keep her daily life busy. One day, she acquires a second-hand Honda Super Cub motorcycle and rides it to school. As time goes by, not only does Koguma have a new adventurous life, but she also forges precious friendships thanks to her precious little motorcycle.

Dog Days takes place in the world of Flonyard, an alternate Earth inhabited by beings who resemble humans, but also have the ears and tails of specific animals. The Republic of Biscotti, a union of dog-like citizens, has come under attack by the feline forces of the Galette Leo Knights. In an effort to save Biscotti, Princess Millhiore summons a champion from another world in order to defend her people. That champion is Cinque Izumi, a normal junior high student from Earth. Agreeing to assist Biscotti, Cinque retrieves a sacred weapon called the Palladion and prepares for war. In Flonyard, wars are fought with no casualties and are more akin to sports competitions with the goal of raising money for the participating kingdoms. Cinque is successful in his role as Biscotti’s champion, but learns that a summoned champion cannot be returned to their home world. The scientists of Biscotti will endeavor to find a way for Cinque to return home, but until they figure something out, he must serve Princess Millhiore by continuing to fight as Biscotti’s hero.

Shortly before the start of a new semester, Misaki Fujioka visits her twin sister Mei Misaki in Yomiyama City. The girls make full use of the last days of summer, roaming around a heat-weary town and visiting various places including a shopping center and shooting stall. When they prowl around her basement, Mei expresses uneasiness about her new class, which is said to be cursed. Craving more entertainment, the twins decide to pay a visit to the local amusement park. But the leisure of a sleepy summer day could soon turn woeful as Mei sees the color of death on her sister—an unmistakable omen that a tragedy is bound to strike. [Written by MAL Rewrite]

The film opens with Nefeltari Vivi flying with Pell in a brief flashback. Returning to the present, Vivi and the Straw Hats meet Crocodile's subordinate Mr. 2 Bon Clay. Mr. 2 shows the Straw Hats his devil fruit ability, which allows him to assume the form and voice of anyone whose face he has touched. Vivi describes a brief history on how the Baroque Works leader Crocodile has used Dance Powder while posing as the country's hero. He has also tricked the rebel and royal armies into fighting each other. Once in Alabasta and after crossing the desert, the Straw Hats find the rebel's base deserted, while the rebel army, led by Vivi's childhood friend Koza, witnesses the port town Nanohana being burned by members of Baroque Works disguised as soldiers of the royal army. The rebels decide to attack Alubarna, where at the same time Mr. 2 impersonates the king, Nefeltari Cobra, and orders the royal army to engage. Meanwhile in the desert, the Straw Hats are intercepted by Crocodile and his partner, Ms. All Sunday. Crocodile aims for Vivi, but Luffy stays behind and distracts him, while the other Straw Hats escape. In the ensuing fight, Crocodile defeats Luffy by impaling him through the chest with his hook and buried alive.