An 18-year-old motorcycle enthusiast, Shogo Yahagi is the main protagonist of Megazone 23 Parts I and II. He has a part time job working at a local McDonalds, and lives a fairly care-free life with his friends. He has a girlfriend by the name of Yui Takanaka, who he meets after nearly running her over.

However, this all changes after an old friend of his shows Shogo a prototype motorcycle with the ability to transform into a robot. During this exchange, they are found out an agent by the name of B.D.. Shogo manages to escape with the prototype.