After receiving an invitation from Dr. Snoozemore, Princess Peach and chums head out for a holiday on Pi'illo island. Dr. Snoozemore is researching a civilization that used to inhabit the island. Luigi falls asleep on an ancient bed, which opens a portal between the real world and the dream world. A vampiric creature appears and carries Peach into the dream world. Mario and Luigi end up on another journey to save the Princess, and the Pi'illo.
The gameplay takes part in both worlds. In the real world, Mario and Luigi move in an isometric perspective, exploring the environments to find pillows to the dream world. Here, Luigi will fall asleep, Mario will enter the portal, and the game will take a 2D side-scroller perspective. Luigi still partners him, but he takes the form as “Dreamy Luigi”. This Luigi has different abilities that are vital to progress through the dream world. The powers are numerous including a tower of tiny Luigis, a Luigi tornado giving a gliding ability, a rolling ball, and the ability to switch gravity in four directions. You activate some of these abilities by interacting with the bottom touch screen; the action depending on the ability. For example, you can pull Luigi's moustache, rub his nose to make him sneeze or spin his bed around.
These two worlds help mix up the gameplay. You still fight battles in each world, but the different abilities mean they have a different feel. When you have Dreamy Luigi, he merges into Mario, replacing the brothers moves with Luiginary Attacks. Mario's hammer or jump attacks spreads out a barrage of Luigi's to attack multiple foes.
Timed button presses are required to perform attacks or counter-attacks. Each enemy has a completely different set of attacks that must be learned in order to properly dodge them. The special moves trigger a mini-game which also require several timed button presses, or using the motion control of the 3DS.
There are a few special boss battles; Giant Battles. You will battle with a giant Luigi, and interact with the touchscreen, with your 3DS turned sideways. These battles also require some learning and timed taps and swipes of the touch screen. I felt these battles seemed to drag on for a bit too long; often 15-20 minutes. It was particularly frustrating if you fail them and need to start the battle again.
If you die in battle, you are given the option to retry on easy mode if you wish. Additionally, you are also given a description of all the enemies attacks, giving you hints how to counter them. It's a nice option to have, but you don't have to use it. This feature doesn't apply to the Giant Battles, and I think that is an oversight because I probably would have used it on a few of them.
There's a few optional side quests, and a few opportunities for mini-games. These are often challenges based on your special moves, or defeating enemies with a certain number of turns. I didn't care much for these. There are hundreds of beans to find, each one boosting one of your stats by one.
I think the game is way too long and doesn't justify the near 40 hour length. The story is pretty basic and the game wasn't as interesting or humorous as the initial games in the series. You go for long periods without seeing or hearing about the main antagonists, so the story seems poorly paced. Many of the side characters aren't very interesting and some of the French-style dialogue was cliché. There's definitely many areas which could be cut from the game since there are loads of optional Dream World segments where you enter, jump over a few platforms, then you've finished the section. I definitely think the game would be better if it was at least half the length.