Serenity's videotape was an 8 mm videotape that Serenity Wheeler (Shizuka Kawai) sent to her brother Joey (Katsuya Jonouchi). It contained a recording of Shizuka, who wanted to speak with Jonouchi before losing her eyesight.
In the manga, Jonouchi received the videotape in the mail shortly after Yugi Mutou received a videotape from Maximillion Pegasus, which was used to conduct a Shadow Game. Jonouchi was concerned that this tape might also be from Pegasus and went to Yugi's house to play it, since he didn't have an 8 mm player himself.
Yugi, Anzu Mazaki and Hiroto Honda watched the video with Jonouchi. They were surprised to find out that Jonouchi had a sister. They asked what Shizuka meant when she said that she had hoped to see Jonouchi one last time and he explained to them about her losing her eyesight. As Jonouchi got upset over not being able to do anything for her, Dark Yugi suggested that Jonouchi come to Duelist Kingdom with him to win the prize money to pay for her operation.
Joey received the video in the mail and watched it alone at home. He decided to take part in Duelist Kingdom to win money to pay for an operation to save Serenity's operation afterwards. He didn't tell his friends about this until much later.