We save Mushroom Kingdom in New Super Mario Bros U
Ian and Anthony start out by playing a few basic from the Layer-Cake Desert, Frosted Glacier, and Sparkling Waters world. After a particularly difficult level in the Frosted Glacier world, they decide to switch things up by allowing Ian to use the GamePad to place blocks to help or hinder Anthony, who is navigating the level. After a couple levels of this, Anthony decides to try Boost Rush mode's Easy course pack, with Ian trying to troll Anthony for most of it.
Ian is playing as Luigi and Anthony is playing as Mario. They decide to go to the first level as a warm-up, but on their way there, they were caught by a Hammer Bro, leading them to a level where they must defeat two Hammer Bros. Anthony quickly kills his, but Ian loses all four lives failing to kill his Hammer Bro. Back on the level selection map, Anthony leads Ian to another warm-up level where they must stomp on Goombas. Anthony, again, quickly stomps on all his Goombas. Ian has more success, but his controllers die before he can kill the last Goomba. They either replace the batteries or get Ian a new controller and complete the challenge. Ian opens the chest in the middle and gets a Super Star added to their inventory.
Back on the level selection map, Anthony accidentally uses a Super Star on both him and Ian. They enter a level marked by a red balloon with a Mii's face. They enter a desert-themed level. Ian jumps on a platform and Anthony, still invincible, kills a Pokey, but is stuck in quicksand. Trying to escape, Anthony hits an invisible block, which spawns a Yoshi egg. Anthony jumps onto the block, breaks the egg, and rides the Yoshi. The two enter a pipe and are led to a cavern. They cross a bridge, kill another Pokey, and obtain Fire Flowers (by this time, their invincibility had worn off). Ian and Anthony continue making their way through the underground level. Anthony uses Yoshi to gobble and spit Ian out, much to his enjoyment. He does it again and spits Ian in the path of a Pokey, disabling his pyrokinesis and sending Ian to his death. Anthony continues on and presumably completes the level.
Ian and Anthony play the next level, Fire Snake Cavern. They enter another pipe, leading them to a pitch-black cavern where the only light comes from light sources (such as fire and lanterns) and an oval of light surrounding Mario and Luigi (who has cryokinesis from obtaining an Ice Flower). Anthony breaks a ? block and out pop an egg with two Baby Yoshis. Ian tries to pick up one as Anthony did, but is hit with fire from a Piranha Plant, losing his cryokinetic abilities. Carrying the Baby Yoshis, the duo moves on to some tilting platforms partially illuminated by lanterns. Ian ditches his Baby Yoshi sometime after the two regain their cryokinetic abilities.
The two of them make their way through the cavern, making their way to another set of tilting platforms. Anthony is hit and killed by a couple Hammer Bros. Anthony eventually respawns and they enter a warp pipe into another part of the cavern. Anthony accidentally kills himself again by falling onto another Hammer Bro. Ian, despite Anthony's protests, ditches Anthony's Baby Yoshi and warps to another area.
The duo go to the Frosted Glacier world in the level Freezing-Rain Tower, during which they will have to fight the boss character Boom Boom. During the first bit of the level, Ian and Anthony have to dodge falling icicles. Anthony easily runs up a slope, dodging several icicles along the way, but is hit by a giant icicle when he reaches the top, disabling his Ice Flower. The two make their way further up the level, dodging more icicles along the way. At one point, Anthony narrowly dodges a giant icicle and throws a frozen Dry Bones at Ian.
They move on onto a platform that rises up and Ian and Anthony must dodge icicles and Dry Bones, as well as ceilings, as the platform rises upward. At the top, they enter a large door to fight Boom Boom. Ian, still with cryokinetic powers granted by the Ice Flower, proceeds to spam balls of ice at Boom Boom while Anthony repeatedly jumps on him (the boss). Boom Boom tries to punch Ian, who jumps out of the way of the attack in time. Boom Boom continues to concentrate his attacks on Ian, and eventually succeeds in disabling the Ice Flower's granted cryokinesis, but was defeated in the process.
Before they can select another level to play, the camera pans over to Peach's castle, where Bowser's airship orbits around it, as does a cloud of purple mist. Ian and Anthony watch as the mist suddenly thickens as the camera switches back to Mario and Luigi. Ian and Anthony begin the level Icicle Caverns. Soon after starting, Ian is crushed and killed by a giant icicle that took him by surprise. Anthony enters a warp pipe and Ian soon appears in a respawn bubble that Anthony pops with fireballs. Ian narrowly dodges being killed again by falling icicles and a Buzzy Beetle and runs to Anthony.
Later in the level, Anthony pops a bubble containing a Mushroom and saves it for Ian. Ian tries to run after it, but the Mushroom falls off a cliff and Ian falls with it, the ice making it near-impossible for him to turn around fast enough. Anthony soon falls off as well, and a montage plays of Ian and Anthony falling off at random times throughout the level. After what appears to be much trial and error, Anthony finally completes the level, but only managed to collect one out of three Star Coins.
Ian and Anthony decide to switch things up: Ian will be using the GamePad and will place random blocks to help or hinder Anthony, who will be playing as usual. They choose the Sparkling Waters world level Waterspout Beach. Ian experiments with placing blocks, most of which are placed to help make it much easier for Anthony to navigate the level. Soon, however, he starts placing blocks in more random locations much more quickly, sometimes helping and sometimes trolling Anthony. Ian was especially unhelpful when Anthony was trying to reach a mushroom in a clam's mouth, but Ian was placing blocks to block him. Soon, Ian is simply trolling Anthony in every way he can, but Anthony still manages to reach the end flag.
The two play another level titled Tropical Refresher. In an underwater portion of the level, Ian immediately proceeds to hinder Anthony's progress by placing blocks above and below him, making Anthony hit a Cheep Cheep and lose his pyrokinetic abilities granted by his Fire Flower. Anthony manages to grab another Fire Flower as Ian continues to troll him. Anthony eventually loses his Fire Flower and his mushroom to unknown causes, as well as being finished off by a Big Urchin.
Anthony tries again, despite Ian's attempts to prevent this, manages to swim over the Big Urchin, but is soon killed by a Cheep Cheep. They try a game mode called Boost Rush, in which players must complete a stage in a specific time limit. Anthony chooses to play as Blue Toad and the Easy course pack, knowing that Ian will continue to troll him. The first level, Acorn Plains Way, starts to speed up as Anthony tries to balance keeping up with the side-scrolling, avoiding being killed by enemies and pits, and avoiding being trapped by Ian's blocks. Anthony finds a small, surprise underground area with a lot of coins, but the tight space makes it easier for Ian to trap and kill him. Fortunately, Anthony respawns in a bubble and eventually completes the level.
They move on to the second course of the Easy course pack, Stone-Eye Zone, and Ian helps Anthony navigate the level. Soon, however, Anthony is trapped in quicksand and is killed by the side-scroll. He respawns, but is soon killed again by falling into a pit after trying to grab a Fire Flower, and dies again after popping his respawn bubble. Anthony dies a fourth time after Ian miscalculated where to place a block, and dies a fifth time as well, losing his last life and this failing the level.
While recapping the episode, Ian and Anthony "learned" the following things: