In 2015, with the greeting "One in a million, We are TWICE!", a new K-pop group stepped onto the world stage. From chart-topping anthems like "CHEER UP," "TT," "What is Love?," "FANCY," and "Feel Special," TWICE captured hearts far beyond Korea, becoming a global phenomenon. This documentary follows their decade-long journey—countless hours of dedication, the dazzling spotlight of worldwide tours, and candid reflections on their growth, friendship, and even their dreams for the future. For ten years, brighter than any stage light, 'ONCE' has been at the very heart of their story. Through never-before-seen interviews and the heartfelt process of preparing a 10th anniversary celebration, the film reveals what TWICE has truly meant to one another—and to the fans who have walked beside them. On their 10th anniversary, 〈ONE IN A MILL10N〉 comes to the screen, revealing the true reason behind the existence of TWICE—a group like no other.