Lincoln and the Louds are ecstatic to welcome their new Gran-Gran, Myrtle, into the family with a tropical wedding celebration; but the festivities are cut short when an old nemesis from Myrtle's secret agent past appears on the island.
While working underground to fix a water main, Brooklyn plumbers—and brothers—Mario and Luigi are transported down a mysterious pipe and wander into a magical new world. But when the brothers are separated, Mario embarks on an epic quest to find Luigi.