When a mysterious portal opens in the Mushroom Kingdom, Mario, Luigi, and many of its residents are swept away to Concordia, a world once bound together by a great Uni-Tree until a dark force shattered it into islands adrift across a vast sea. Aboard the mobile Shipshape Island, the brothers befriend Connie and the pig-like Snoutlet and set out to reunite the scattered lands, seeking each island's lighthouse to reconnect it to their new home. As they journey, they uncover the sinister power spreading isolation across Concordia and must draw on their bond to bring the fractured world back together.