One-off manga collaboration between the writers of the titular, immensely popular franchises, Akira Toriyama and Eiichiro Oda. Purely an exercise in curiosity, the manga sees characters from both worlds placed together in what, in all honesty, barely passes as a story.
Both mangaka are huge fans of each other's work; Oda, being originally inspired by the amazing Dragonball works, and the retired Toriyama, who, through his kids' interests, discovered the grand pirate adventure One Piece. The two writers decided in 2007 to collaborate and create a short strip with their most famous characters joining forces with each other. Thus the twenty page DragonBall x One Piece: Cross Epoch was born. What resulted was not in any way related to the story arcs of the original series.
It is certainly best approached as a novelty. There are no Super Saiyan battles or adventures to exotic islands. There is hardly any action or plot to speak of at all. However it is clearly targeted at fans of the series, or possibly attracting fans of one series to the familiarity of the other.
Primarily comprised of a few barely connected scenes, Cross Epoch begins with Mr. Satan becoming king of a fictional world while other character pairings head to an undisclosed place. (If this all sounds rather vague, that's because it is in the manga too). Krillin and Chopper are placed together as policemen on a train, while Nami and Bulma appear as spaceship pilots. Following this, Sanji and Master Roshi are cast as two cowboy law-enforcers on a dusty road. (If this all sounds rather haphazard, that's because it is in the manga too). Captain Vegeta flies past on his pirate ship, complete with oars and sails, dressed like an emperor out of Dune. His crew consists of Nico Robin, Trunks, and Usopp. Piccolo and Zoro have a page together — arguing, no less — before the two heroes finally make an appearance. Goku and Luffy discover the floating ball spaceship of Emperor Pilaf and Buggy the Clown, before destroying it. Even Oda's recurring Easter-egg of a character, Pandaman, makes an appearance in the corner of a panel. Finally all the characters end up at the intended location in front of the dragon Shenron who grants their wish — the creation of a giant cake mountain and a river of tea that they can all share.
Now, if this all seems to you like a Shonen Jump marketing ploy that ended up with the story depth and substance of a child's learn-to-read book, I wouldn't say you were too far off. Except, judging by the sense of humour throughout the series and of the writer's themselves, I would suspect it more of being a fun "what-if" situation that is not intended as anything further. Think of it as a fan-fiction offering, except written by the original writers. Hardly essential reading, yet interesting enough to see newcomer One Piece levelled with Dragonball's legendary status.