Japanese authorities faced their first large cluster of COVID-19 infections in early February when the Diamond Princess luxury cruise ship returned to port in Yokohama. Of the 3,700 passengers and crew, more than 700 were infected with the novel coronavirus, and 13 people died. NHK conducted an in-depth investigation into what happened inside the ship. It reveals a microcosm of Japan's battle against the virus, and offers important clues for managing the crisis.