Two Plumbers and a Baby
In the Kingdom of Youth, Mario, Luigi, Toad and Princess Toadstool are all walking down a path wielding magnifying glasses looking at footprints, with Mario stating that King Goo Goo Ga Ga Koopa is somewhere in the kingdom, planning to cause mischief with the Kingdom of Youth's Fountain of Youth. Eventually, Toad finds the end of the trail of footprints, which are revealed to be nothing more than paper-thin pads; as soon as Toad informs Mario, Luigi and Princess Toadstool that they have reached the end of the footprint trail, an old jalopy, driven by three old men, all of whom are wearing clothes commonly associated with teenagers, drives by rather noisily. Stopping their jalopy, the three old men exit it and after proclaiming they are going to the Fountain of Youth, jump down a nearby slide.
Hearing the old men talk about the Fountain of Youth, Mario, Luigi, Toad and the Princess decide to follow them down the slide, with Mario hoping that there will be food at the bottom of the slide. After some sliding, the three old men are, somewhat roughly, deposited near the Fountain of Youth, with one of the old men saying that the fountain will make him and his companions eighteen again. As the three old men approach the fountain, eagerly saying how soon they will be 18 again, King Goo Goo Ga Ga Koopa, snickering, says to himself that soon they will be 18 inches tall and work for him after he turns the fountain up to full blast. Koopa turns a nearby dial attached to the Fountain of Youth, which causes a large amount of water to flow into it.
Not noticing the excess amount of water pouring into the Fountain of Youth, the three old men jump into the fountain and begin to splash each other; after some splashing, the men are suddenly changed into babies, who crawl out of the fountain and are quickly grabbed by King Goo Goo Ga Ga Koopa. Taunting the babies, Koopa has his nose grabbed by one, causing him to yelp in pain, dropping the babies as he soothes his nose.
Realizing that he has dropped the babies, who are now running away, Koopa calls for his Albatoss minions to appear and capture the fleeing babies. Hearing Koopa's command, several Albatosses, all of whom are brandishing Bomb Nets, appear and begin to drop their nets on the babies. Approaching the fleeing babies, the Albatosses drop their nets on them, which fall on to and trap the babies, allowing Koopa to capture them with little effort.
Mario, Luigi, Princess Toadstool and Toad, having recently been deposited from a slide, notice, from behind some nearby pillars, King Goo Goo Ga Ga Koopa capturing the children, who Mario realizes are the three old men who had been encountered by him and the others earlier. As Koopa enters a nearby fortress, carrying the three babies, Luigi hears a noise emanating from the fortress. As Luigi presses his ear against a wall of the fortress, Luigi is instructed by Mario and Princess Toadstool to look in a window of the fortress with them. Inside the fortress, Mario, Luigi and the Princess see several babies doing a variety of tasks, such as carrying logs and polishing a floor, all under the supervision of Koopa Troopas.
As Mario, Luigi and the Princess relent that what King Goo Goo Ga Ga Koopa is forcing these babies to do is terrible, Toad, who is behind Mario, suddenly trips, knocking all four of them into a wall of Koopa, which happens to be made of numbered blocks, knocking it down. Hearing the wall collapse, Koopa notices the good guys lying on the floor and leaps on to a nearby, wheeled rocking horse, which resembles an Ostro and begins to wheel after them. As Luigi, Toad and the Princess run from Koopa, Mario, grabbing a block, which fell from the collapsed wall, throws it at Koopa, causing him to crash his rocking horse.
Catching up to Luigi, Toad and Princess Toadstool, Mario begins to run from Koopa's fortress with them; unfortunately, as she runs, the Princess trips and falls into the nearby Fountain of Youth and begins to flail about in its deep water. Before the Princess can drown, however, Mario, Luigi and Toad manage to pull her out of the fountain, only to watch, dumbstruck, as she transforms into an infant right in front of them.
While staring at the now juvenile Princess Toadstool, Mario, Luigi and Toad are approached by King Goo Goo Ga Ga Koopa, who is still riding his rocking horse. While Koopa taunts Mario, Luigi and Toad, the Princess, reaching into Mario's overalls, pulls a plunger from them and quickly throws it at Koopa's face, which the plunger becomes stuck to.
While Koopa begins trying to pull the plunger off his face, Mario, grabbing Princess Toadstool, begins to run away with Luigi and Toad. As they run, Koopa, managing to pull the plunger off his face, orders his Koopa Pack, which consists of Albatoss wielding Bob-Ombs, to attack the good guys. As Koopa chases the good guys on his rocking horse, he orders his Albatoss minions to use their Bomb Nets on them; following Koopa's order, an Albatoss drops a Bomb Net, which explodes and releases a net, which misses the good guys by several feet; however, the net does succeed in causing Mario to trip, which causes him to accidentally fling the Princess through the air. As the Princess flies happily through the air, she is caught by Luigi before hitting the ground; with the Princess safely in his arms, Luigi reprimands Mario, saying he should be more careful with the Princess. As Mario, Luigi, Toad and the Princess continue to run from him, Koopa becomes entangled in a net dropped by an Albatoss and is left furiously trying to escape.
As Koopa is left trying to untangle himself from the net, Mario, Luigi, Toad and Princess Toadstool reach a nearby playground. After arriving in the playground, Mario quickly instructs the others to go on to two nearby seesaws; before Mario can enact a plan, though, he is attacked by a Koopa Troopa riding a rocking horse and wielding a large safety pin. Managing to evade the attacking Koopa Troopa, Mario accidentally slips on a nearby roller skate and falls on to the opposite ends of the seesaws Luigi, Toad and the Princess are standing on, which causes them to be flung through the air. After soaring through the air for a small amount of time, Luigi, Toad and the Princess safely land on the other side of a large wall.
In the playground, Mario is in the midst of running from several Albatosses, who are dropping Bomb Nets and Bob-Ombs on him. While running from these Albatosses, Mario notices a rope hanging from a tree and uses it to swing through the air and over a wall, on the other side of which he meets Luigi, Toad and Princess Toadstool. Greeting them, Mario tells them to run, saying that they will save the babies that King Goo Goo Ga Ga Koopa has captured later on, at night.
Later, Mario, Luigi and Toad, who are in a large field, are pushing Princess Toadstool on a swing, with Mario complaining that he knows about baby brussel sprouts, mini frozen pizzas and tiny tortellinis, but nothing about babies. Hearing Mario's complaints, Toad reassures him, saying that taking care of the Princess should be easy, as all they need to do is feed her when she's hungry and wash her when she's dirty. After Toad finishes talking though, Luigi is shocked to find that the Princess is no longer on the swing. Panicking, Mario, Luigi and Toad, after looking around, see the Princess crawling along the branch of a nearby tree, trying to catch a green bird that is sitting on the end of the tree branch. Fearing that the Princess will fall from the tree branch she is on, Mario begins to climb up the tree after her. Getting on the branch the Princess is crawling on, Mario tries to persuade her to crawl over to him; unfortunately, as Mario tries to tempt her to come over to him, saying he has a big surprise for her, the branch they are on breaks.
Sent tumbling towards the ground, Mario lands in a pile of leaves, while the Princess bounces off a branch of the tree. Rushing to Mario, Luigi and Toad point out to him that the Princess, who is now holding a twig, has bounced directly into a nearby canyon with a river at the bottom of it. As she falls into the canyon, giggling, the Princess' diaper gets caught on a tree branch sticking out of a wall of the canyon. Seeing that there is still a chance to save the Princess, Mario, using some tree branches and a large leaf, creates a hang glider and uses it to swoop down into the canyon where the Princess is. Getting close to the Princess, Mario grabs her and begins to glide back up the canyon; as he glides, though, the Princess begins to tickle him, which causes him to laugh so hard that he loses his grip on his hang glider and is sent plummeting down into the canyon below, while the Princess safely descends into the canyon, still hanging on to the hang glider.
Luckily, Mario survives his fall into the canyon by tumbling into a large pile of mud; shortly after he does so, the Princess lands next to him and proclaims that he is a lot of fun to be with. Later, as Mario washes some mud off himself by a waterfall, Toad tells him once again that babysitting is not too hard and keeps watch on the Princess, who is sitting on a nearby raft. Wanting to keep the Princess occupied and out of the river her raft is on, Toad gives her a pinwheel; once she gets this pinwheel though, she blows on it with enough force to move the raft down the river. As the Princess is propelled down the river, with her raft being slowly eaten by Trouters, Toad, panicking, yells for Mario and Luigi to help him save the Princess from being eaten by voracious Trouters.
As she flows down the river on the raft, Princess Toadstool, oblivious to the danger surrounding her, grabs a Trouter from the river and throws it towards Mario, Luigi and Toad; attempting to flee this Trouter, Mario and Luigi climb a tree to escape it while Toad runs from it. This tree offers little aid to Mario or Luigi though, as the Trouter simply chews through the trunk of it, causing the tree to fall into the river. As the brothers sail down the river on the collapsed tree, Toad, using several rocks as stepping stones, manages to hop on to the tree they are on. As they sail down the river on the tree, the brothers, after being alerted by Toad to the nearby Princess, whose raft is almost been entirely eaten by Trouters, take out their plungers and use them as oars to pull alongside the Princess' raft. Once close enough to the Princess, Mario, after tying a rope to his plunger, uses it to grab the Princess and pull her in to the arms of Luigi. Toad then complains that there isn't much sitting in babysitting, but Luigi assures him that there will be once they get her to take a nap.
Later, somewhere in the Kingdom of Youth, Mario, Luigi and Toad are in the middle of rocking the Princess back and forth on a hammock, with Mario singing lullabies to her. When Mario finishes his lullaby, he, Luigi and Toad, briefly watching the sleepy Princess (while reminding each other to not to disturb her), begin to walk away from her, only to be forced to rock her back and forth again when she begins crying. Sometime later, Toad finally manages to calm the Princess down by amusing her with two fireflies he has captured; after leaving her with the fireflies, Toad walks over to the nearby Mario and Luigi and tells them that Princess Toadstool isn't that hard to take care of and motions towards her, only for him, Mario and Luigi to see that she is gone, having followed the fireflies all the way to King Goo Goo Ga Ga Koopa's castle.
In the front of Koopa's castle, the Princess is crawling along the ground near the Fountain of Youth, chasing several fireflies. Having caught up to the Princess, Toad sees that she is going to fall into the fountain and quickly has Mario rush over to her and pick her up before she falls into the fountain again. It is only when he gives the Princess to Toad that Mario sees that King Goo Goo Ga Ga Koopa has taken the fountain's controls. As Luigi bemoans the loss of the controls, saying without them, Princess Toadstool will never be returned to normal, Mario cheers him up, saying that all that needs to be done to repair the fountain is to reverse the flow of its water. After saying this, Mario goes over to Koopa's castle and opens a door on it, exposing a room filled with pipes.
After getting all the pipes they need, Mario and Luigi, after some work, make a large piping system extending from the fountain. Before either Mario or Luigi can activate this piping system, though, King Goo Goo Ga Ga Koopa, riding an Ostro rocking horse, barges out of his castle towards the brothers. As Koopa barrels down towards them, Mario and Luigi, grabbing a large pipe, use it to catapult Koopa through the air and into the Fountain of Youth. After crashing into the fountain, Koopa emerges from the fountain as an infant version of himself and storms off, sobbing and vowing revenge on Mario and Luigi.
With King Goo Goo Ga Ga Koopa defeated, Mario turns on his piping system, which causes the dial on the Fountain of Youth to reset itself; it also causes the water in the fountain to flow backwards. As the fountain flows in reverse, all the children Koopa had captured surge forth out of his castle and leap into the fountain, while Toad lowers Princess Toadstool into it. Upon touching the water of the fountain, the Princess reverts to her normal form, as do all the other surrounding babies, who, upon turning back to normal, exclaim that they are happy to be back to normal and to be able to stay up until 10pm again.
With Koopa vanquished from the Kingdom of Youth and everything in it returned to normal, Mario, Luigi, Toad and Princess Toadstool decide to leave the Kingdom of Youth on several rocking horse Ostros; as she rides behind him, the Princess says to Mario that if he ever decided to quit his job as a plumber, he could become a babysitter, to which Mario responds that the only sitting he wants to do is to a big plate of pasta and finally the four laugh.
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