Caught with a severe fever before his school trip, Tobio Ikuse, a second-year high school student was forced to miss out his school trip with his schoolmates on a luxury liner. Four days later, the news about the disappearance of the teachers and second-year students reaches Tobio's ears.

Few months after the incident, Tobio having slowly recovered from the shock accidentally encounters one of his classmates again.

(Source: High School DxD Wikia, edited)