It’s 1990 and Aoi is looking out at a river and feeling sad for herself – the boy she loves just moved to America. She’s wearing his hat, but just then a just of wind comes up and blows the hat into the river, hundreds of feet below. She makes the instinctive move to reach for the hat when she’s stopped by a man about 10 years older than she is, who asks for her help.

This man and three of his friends are from the year 2006, and Aoi thinks one of them is familiar…