Okay so... Two challengers appear, they battle, Mario wins, he takes pity on Kirby and releases him, and they form an alliance to ensure that the torturous games Master Hand puts them through will end. About Metaknight, he may possibly be being controlled in some way. Manipulated, also his ship may have been the vessel for those monsters. Although it does seem like they are coming from the ground. I don't believe it will be an in depth story mode. Probably just some chatter (Maybe) from the characters or the entire thing will be displayed through cutscenes between battles (Like the Giga Bowser transformation in Melee.)