Plot
Mario and Princess Peach are strolling near Peach’s Castle when a mysterious thundercloud (possibly Lakithunder) appears and strikes the castle with lightning. Mario goes to investigate, but during the distraction, Bowser Jr. sneaks in, kidnaps Peach, and flees. Mario immediately gives chase as Bowser Jr. drags her away. While pursuing him, Mario goes off-screen, takes a hit, and reverts to his smaller form. Bowser Jr. escapes further, and the chase continues.
In the intro sequence that plays if the title screen is left idle, after Mario begins chasing Bowser Jr., he retreats from an incoming green Koopa Shell and is struck off-screen. He then resumes the chase after leaping to knock the game’s logo back into place.
The first world’s boss is unusually Bowser himself. After Mario activates a switch that collapses the bridge beneath him, Bowser falls into lava. His flesh burns away, leaving him as a skeletal version known as Dry Bowser. Later, in World 8, Mario battles this undead form again as the penultimate boss. Though Dry Bowser is defeated and falls into a pit, Bowser Jr. later retrieves his remains. By casting them into a cauldron filled with a magical potion, he revives Bowser who returns at a massive size, nearly ten times larger than before.
In the finale, Bowser and Bowser Jr. confront Mario together. Despite their combined efforts, they are defeated once more and sent tumbling into another pit, allowing Peach to be rescued. During the closing sequence, Bowser Jr. is shown dragging his unconscious father across the castle floor. Upon noticing the player watching, he yells something before continuing his task, bringing the game to its conclusion.