Rolling Down the River
Princess Toadstool has been captured by Captain Koopa, who is racing down the river of River World in his ship, the Sinister Star. As Captain Koopa paces back and forth near the steering wheel, First Mate Mouser appears from below deck to report that he has found no trace of any stowaways. Not satisfied with this, Koopa violently shoves Mouser aside and ventures below the deck of the Sinister Star.
Down below decks, Mario and Luigi, wearing trench coats and fedoras, are in the middle of a game of Go Fish against two Koopa Troopas and a Goomba. Winning the game, Mario proceeds to grab and count the large pile of coins he won. As Mario continues to count his coins, Luigi whispers to him, asking how much longer they need to distract the Koopa Troopas and the Goomba; Mario whispers back that they need to keep them distracted until either Toad finds and releases Princess Toadstool or he wins more of the Koopa's money.
Up above, on another floor, Toad, carrying a silver platter and disguised as a waiter, approaches a locked door. Toad proceeds to tell the door's Goomba guard that he is here to deliver a birthday cake to the captive Princess Toadstool and proceeds to show the Goomba a rather poorly made cake. Suddenly, Toad is grabbed from behind by Captain Koopa and drops his cake on the ground, revealing various instruments of escape hidden in it, which includes a file, a hacksaw and two hand drills. Opening Princess Toadstool's cell door, Koopa throws Toad into the cell near the Princess and proceeds to tell the Goomba guard to be on the watch for Mario and Luigi.
Above deck, Mario has just won another game of cards, while Luigi is begins to worry about Toad. Calming Luigi down, Mario notices a Goomba hop by with a plate of chicken on its head; running up to the Goomba, Mario grabs a piece of chicken off its plate and takes a bite of it, though in the process, lets several cards fall out of his coat sleeve. A Koopa Troopa, seeing the fallen cards, approaches Mario and accuses him of cheating and struggles to pull his coat off, revealing several more hidden cards. Managing to pull Mario's coat off, the Koopa Troopa realizes it was Mario he was playing against the entire time; as Mario and Luigi begin to run away, the Koopa Troopas and the Goomba give chase to them.
The chase continues until Mario and Luigi are cornered on the top level of the ship. At this point, the brothers see no other alternative than to jump overboard into the river to escape the pursuing Koopa Troopas and Goomba. In the river, the Mario Brothers are suddenly attacked and chased by a knife-and-fork-wielding Trouter. Trying to escape the Trouter, the brothers suddenly get caught in a lasso and are dragged aboard a ship; once on the ship, they meet Mark Twang, the captain of the Ding Bell. Befriending Mark Twang, the Mario Brothers tell him of Princess Toadstool's capture by Captain Koopa and also learn that Mark Twang is after the Sinister Star as well; it seems Mark had entered a race against Koopa, with whoever winning the race becoming the King of the River. Unfortunately, it seems Koopa had sabotaged the engine of the Ding Bell, wrecking it almost completely. Deciding to help Mark, Mario and Luigi decide to repair the Ding Bell's engine.
Onboard the Sinister Star, Captain Koopa is in the process of taunting Princess Toadstool and Toad while First Mate Mouser keeps lookout with a telescope. Seeing the Ding Bell, Mouser proceeds to alert Koopa of this. Grabbing the telescope from Mouser, Koopa, wondering how Mark Twang fixed his engine, sees Mario and Luigi riding the Ding Bell; furious, he throws his telescope to the ground and stomps on it.
Steering the Ding Bell, Mark Twang informs Mario that they need more speed to catch up to Captain Koopa and the Sinister Star; through an intercom, Mario manages to tell Luigi, who is in the engine room, to increase their speed. Onboard the Sinister Star, Captain Koopa orders First Mate Mouser to bring him a Black Pit Bob-Omb; hesitantly, Mouser brings forth a chest containing an enormous Bob-Omb and passes the explosive to Koopa. Lighting the fuse of the Black Pit Bob-Omb, Captain Koopa hurls it into the river near the Ding Bell, where it creates a monstrous explosion which, in turn, creates a large, gaping, black pit.
Seeing the black pit, Mark Twang begins trying to steer the Ding Bell away from it, only to be sucked closer to it by the current of the water. As Mark begins to lose hope of escaping the black pit, Mario and Luigi begin to enact a plan; taking out his plunger, Mario ties some rope to the end of it while Luigi wheels a cannon near him. Loading his roped plunger into the cannon, Mario fires it at a nearby, target-engraved rock; the plunger attaches to the rock, anchoring the Ding Bell in place. As Princess Toadstool and Toad cheer for Mario, Captain Koopa begins to taunt them, saying that there is no way the Ding Bell will ever pass the black pit.
Having docked the Ding Bell near the target-emblazoned rock, Mark Twang proceeds to tell Mario and Luigi that they should see the Mouth of the River, a woman who knows everything about River World and who may be able to help them pass the black pit. In a parked caravan, Mario, Luigi and Mark Twang find the Mouth of the River and ask her how they can get the Ding Bell past the black pit; she proceeds to give them some cryptic advice, saying that they should have the river "go around" the black pit. Later, near where the Ding Bell is docked, the Mario Brothers are finishing what appears to be a large, intricate piping system. Turning the system on, the Mario Brothers and Mark Twang watch as a pipe sucks up the water of the river and places it in a passage separated from the black pit by a wall of rocks, using this new river to pass by the black pit.
Despite passing the black pit, Mark still believes he will not be able to catch-up to Captain Koopa and the Sinister Star, which is extremely far ahead. The Mouth of the River, who has joined the crew of the Ding Bell, says that all hope isn't lost and proceeds to give Mario and Luigi a pair of magic pendants, saying they'll help them. Putting the pendants on, the Mario Brothers are transformed into Super Mario and Super Luigi.
Aboard the top deck of the Sinister Star, Captain Koopa and a rather bored-looking First Mate Mouser are on the look-out. Seeing the Ding Bell approaching, Koopa, in his frustration, throws his telescope at Mouser and proceeds to stomp on it again when it lands on the ground. Onboard the Ding Bell, Mario and Luigi, with their new super-strength, are jumping up and down on a pump-like mechanism, pushing the ship forward as the Mouth of the River cheers them on. Pulling-up alongside the Sinister Star, the Mario Brothers leap from the Ding Bell and land on Koopa's ship as their own speeds ahead. As Princess Toadstool, Toad, Mouser and several Koopa Troopas watch, the Mario Brothers effortlessly lift Captain Koopa above their heads. As Toad and the Princess thank the brothers for rescuing them, Captain Koopa suddenly grabs the empowering pendants from around the brothers' necks; without the pendants, they lose their superpowers and regress to normal, and are easily subdued by Captain Koopa.
Afterwards, Mario is forced to walk the plank by Captain Koopa, First Mate Mouser and a Koopa Troopa. As Mario prepares to be shoved into the river, where several hungry Trouters await, the Ding Bell appears and begins to blast the Sinister Star with Turnips, fired by the Mouth of the River from a cannon. As Koopa orders Mouser and a Koopa Troopa to return fire, the driver of the Sinister Star, Clawgrip, is knocked aside by Turnip projectile; with Clawgrip knocked unconscious, Princess Toadstool manages to take hold of the Sinister Star and begins to steer it towards a nearby beach. Charging towards the beach at full speed, the Princess has the Sinister Star crash into it with enough force to send Captain Koopa flying off the ship. Sent spiraling through the air, Koopa plummets towards the sand and lands in it, becoming buried and left vowing vengeance on the laughing Mario, Luigi, Toad and Princess Toadstool.
As the Ding Bell reaches the finish line of the river, Mario, Luigi, Toad and the Princess congratulate Mark Twang on becoming King of the River. After congratulations are done, Mario's group and Mark Twang await their victory lunch, which was made and is being served by the Mouth of the River. As the Mouth gives Mario his lunch, she tells him it's catfish pizza, which he is hesitant to try, as the catfish on the pizza is still raw and uncooked.
Animation and continuity errors
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