The anime depicts an original story before Kaho arrives, when Yuki and Natsu have just met in Okinawa. (Source: ANN)

The promotional video is a home video exclusive. Customers must first purchase all four volumes of the Sex 30th Anniversary Edition manga volumes, take the tickets included in volumes 1-3 and send them with the postcard included with volume 4. The Blu-ray Disc will be sent to buyers in March 2018 so long as the postcard is postmarked before December 31. (Source: ANN)