4x13 - Dragon Ball Z: Dragon Box

Trunks realizes some hidden truths about the fabric of his "parallel world." For example, even if Goku and the others are able to defeat the Artificial Humans, Trunks' own tragic future would remain unchanged. In the future, Bulma believes that it would be nice if there were another, peaceful future, which is why she built the time machine. However, they soon discover that there is a second time machine out there...