The Drum Island Arc, also called the Drum Kingdom Arc, is the tenth story arc in One Piece and the fourth arc of the Alabasta Saga. It involves Monkey D. Luffy, Roronoa Zoro, Nami, Usopp, and Sanji of the Straw Hat Pirates. This arc introduces several important characters: the newest member of the crew, Tony Tony Chopper, Doctor Kureha, Doctor Hiriluk, Dalton, and Wapol, the main antagonist.
The main reason behind this arc, however, is the pressing concern of finding medical treatment for Nami, who suddenly fell seriously ill. The island in question here, Drum Island, is portrayed as a winter island with a harsh, snowy climate, and the uniqueness of the drum-shaped mountains makes it very different and hard to cross.
The plot of the Drum Island Arc begins when the Straw Hat Pirates are in dire need of medical assistance for Nami, who has contracted a potentially deadly illness. While trying to find a doctor that can save her, they happen upon the kingdom of Drum Island, which is very famous for its medicine. They soon learn that the island is embroiled in political turmoil upon their arrival. Wapol, a self-forming selfish and tyrannical king, had fled the island when it was attacked by pirates, taking all but one doctor with him.
The Straw Hats finally meet Dalton, the former captain of the guard and presently elected boss of Drum Island. He says there is only one doctor left, the eccentric Dr. Kureha, living at the top of a treacherous mountain. Thus, to help Nami, Luffy and Sanji proceed with a hazardous climb up the mountain, carrying their companion. This will show how much Luffy can be dedicated to anything and how tough the island is physically.
In the process, he met Tony Tony Chopper, a reindeer who turned into human form with the consumption of the Human-Human Fruit. There are glimpses of Chopper's past with the quack doctor Dr. Hiriluk, who actually is of noble heart and took Chopper in as a surrogate father. Those were emotional touches of who Chopper was, of acceptance and dreams, and finally, what it means to be human.
Things only worsen with the return of Wapol, who becomes hell-bent on taking his throne back. Wapol's return pushes the Straw Hats into a conflict to protect the island and its people from his tyrannical subjugation. This fight is whisked right up to the top of the mountain, pitting Luffy against Wapol in a fight that once again proves just how physically strong Luffy is, as well as his commitment to protecting people in dire need of his help.
Throughout the arc, Dr. Kureha is very instrumental, treating Nami but also detailing more on the history of the island and the legacy of Dr. Hiriluk. Her discussions with Chopper provide insight into his medical education and his struggle to cope between his dual nature as a reindeer and a human.
It ends with the defeat of Wapol by Luffy, which finally relieves the island of fear of his eventual return. This victory, along with the truth about Dr. Hiriluk's dream to have the island bloom with cherry blossoms, makes for a touching moment when Chopper sees his vision realized. Moved by the actions of the Straw Hats and touched by their freedom, Chopper decides to join their crew as their ship's doctor, thus also achieving his own dream: to have adventures and find his place in the world.
The ending of this arc could be called one of the biggest events that have happened in the One Piece storyline up until now. It doesn't just add another person to the Straw Hat crew, but also expands with such themes as dreams, friendship, and the touch of good governance within a series. Chopper himself brought medical knowledge into the crew and added another dimension with his power to stride in both the animal and human worlds. It subtly introduces other plot elements that turn out important later in the series, with brief mentions of characters like Blackbeard and Ace.