About 65 million years ago, a meteorite crashes into the Earth, killing the dinosaurs and splitting the universe into two parallel dimensions. The surviving dinosaurs cross into a new dimension and evolve into a humanoid race. In the present, a mysterious woman leaves a large egg, along with a rock, at a Catholic orphanage, but as she attempts to leave, she is accosted by King Koopa, who demands the location of the rock. Rocks then fall onto the woman, killing her. The egg then hatches, containing an infant girl. Years later, Italian American plumbers Mario and Luigi live in Brooklyn, New York. They are on the verge of being driven out of business by the mafia-operated Scapelli Construction Company led by Anthony Scapelli. Luigi falls for NYU student Daisy, who is digging under the Brooklyn Bridge for dinosaur bones. After a date, Daisy takes Luigi back to the bridge only to witness two of Scapelli's men sabotaging it by leaving the water pipes open. Mario and Luigi manage to fix it but are knocked unconscious by Iggy and Spike, Koopa's henchmen and cousins, who kidnap Daisy. Mario and Luigi awaken and pursue them through an interdimensional portal that leads them to Dinohattan.
