New Super Mario Bros. Wii is a 2.5D side-scrolling platformer; although it plays out in 2D, most of the in-game characters and objects are 3D polygonal renderings on 2D backgrounds.[8][9] In single-player mode, the player controls Mario and must complete various levels, which are filled with both helpful items and harmful obstacles. The player must maneuver him to a large flag pole at the end of each stage to progress.[10] The game can be played with the Wii Remote held horizontally, or vertically with the Wii Nunchuk attached.[11] Mario can run, jump, and perform additional moves returning from New Super Mario Bros. such as wall kicks, ground pounds and double and triple jumps.[12] New Super Mario Bros. Wii frequently makes use of the Wii Remote's motion control features; the player can shake the controller in order to perform various different actions, such as a short spin jump which kills enemies, a mid-air twirl that can be used to sustain air time, and the ability to pick up, carry and throw certain objects. Another advantage of the mid-air twirl is the ability to wall-jump off of a singular wall as long as the mini-mushroom or Ice Flower is equipped. Mario has to wall-jump before mid-air twirling back to the wall to repeat the action.[13][11][14] Certain areas within levels, such as specific platforms, can be manipulated by standing over them and tilting the Wii Remote.[13] Certain levels are set underwater, where the player must swim to traverse the level.[15]
In addition to gold coins, which the player can collect to earn extra lives, levels contain power-ups encased in floating blocks[16] which aid Mario in his quest. For instance, the Super Mushroom makes Mario increase in size and allows him to take one extra hit; the Fire Flower lets Mario shoot fireballs at enemies; and the Super Star gives the player temporary invincibility, increases his running speed, and provides light in any dark level. The Mini Mushroom, which reappears from New Super Mario Bros., causes Mario to shrink in size, letting him jump higher, run on water and fit through small spaces, albeit while making him vulnerable to enemies and other obstacles.[17][8] New power-ups include the Propeller Suit, which allows Mario to fly for a short time by shaking the Wii Remote;[11] the Ice Flower, which gives Mario the ability to shoot balls of ice which freeze enemies into large ice blocks that can be lifted and thrown; and the Penguin Suit, which also gives Mario the ability to shoot ice balls, but additionally allows the player to slide along the ground and across water, as well as giving them tighter control on ice and in water.[11][18][8] Yoshi appears in certain levels and is able to eat, swallow and spit enemies and objects, and flutter for a period of time.[19][20]
The game consists of eight worlds,[9] with a secret ninth world which appears once the main game has been beaten. There are 77 (excluding toad houses and warp cannons) levels in the game, 8 in World 9, which are all initially locked.[21] They can be unlocked by collecting all three hidden Star Coins in each level of their respective worlds. Levels are accessed via a 3D world map;[8] completing a stage unlocks the next one, with multiple paths sometimes available after completing a stage. Some stages have an extra secret goal, which, when reached, unlocks an alternative path for a player, such as a cannon leading to a later world.[22] Each world contains two boss levels — a midway fortress and a castle at the end of the world — where the player battles one of the seven Koopalings. World 4 and World 6 also have a third Airship boss levels, where the player must battle Bowser Jr. to progress to the next world.[13][23] In addition to levels, there are also "Toad Houses" scattered across the map in which the player can play a short minigame to earn extra lives or items that can be equipped from the map screen.[24][25] Map screens often have enemies roaming them in certain dedicated areas which, when encountered, initiate a "mini-boss" fight that always awards the player three mushrooms. At certain points, a Toad will appear trapped in one of the previously completed levels, and the player can choose to rescue him from a block and carry him safely to the end of the stage in order to unlock an extra Toad House.[26] Every course contains three Star Coins which are hidden in hard-to-reach areas.[26] These can be spent on hint movies which show off tips and tricks for the game, including the locations of secrets and methods for finding more Star Coins and collecting lives.[27]
The player begins the game with five lives, but more can be obtained through collecting coins, picking up 1-Up Mushrooms, performing combos,[25] and playing minigames.[23] Losing a life will return the player to the map, and losing all lives results in a game over, forcing the player to return from their last save point.[10] Most levels contain a midway flag which allows the player to return to that point when returning to the level.[10]
New Super Mario Bros. Wii features "Super Guide", a concept meant to help players that are having difficulty completing a certain level, and the first Nintendo game to include the concept.[28] During single-player mode, if a player dies eight times in a row in any level, a green "!" Block appears, which can be hit to allow a computer-controlled Luigi to show the player a safe path through the level without revealing any Star Coin locations or secret exits. The player may interrupt the guide at any time and take control of Luigi from that point. After Luigi completes the course, the player has the option to try the level again, or skip it completely
