Nintendo has shared the plot for New Super Mario Bros. 2, explaining that:
Trouble always seems to find its way to the Mushroom Kingdom! Just as Mario and Luigi go off on a coin hunt in the sky, their beloved Princess Peach™ is kidnapped by the dastardly Koopalings. The brothers have to venture through over 80 stages to rescue her, grabbing an endless deluge of coins across land, water, air, and snow along the way.
Okay, so maybe “plot” is used loosely. This is a Mario game we’re talking about though.