The second game in the Mario & Luigi RPG series, released in 2005 for the Nintendo DS. The Japanese name is Mario and Luigi RPG 2x2.The game changes a few game mechanics from the first one, and teams the brothers up with their time-travelling baby selves (who otherwise appeared in the Yoshis Island games) using hilariously bad time travel "logic." Princess Peach travels to the past and is promptly kidnapped when the Mushroom Kingdom of the past is invaded by the alien Shroobs. Our Heroes go into the past to find her and have to collect the crystal shards that formed the Cobalt Star that was used to power the time machine in the first place while teaming up with their infant selves as well. Other than that, it's still pretty much the same game with witty dialogue and funny animations and some limited usage of the DS's features. The move-sets were altered to give each pair (bros and babies) unique items and moves, such as only the babies using hammers, and the Bros. moves in battle have been replaced by Bros. items instead.
Big Damn Heroes: Kylie Koopa disguises herself as a Shroob and ambushes the ones ambushing the heroes on the mothership.
Boss Bonanza: The end features a minigame fight with the Shroob Mothership, then normal boss fight with Princess Shroob, then her older sister, and then said sister's One-Winged Angel form. THEN there's a Post Final Boss, Shrowser, after that.
Buried Alive: Petey Piranha does this to himself in his battle with him, although he only does it up to his oversized head.
Evil Counterpart: The Shroobs often act like this to the Toads, even having an enemy ingame known as the Shroobsworth.
Evil Detecting Baby: When Princess Shroob arrives in the Mushroom Kingdom disguised as Princess Peach, Baby Peach is seemingly the only one to recognize her as she cries right in front of her.
Also, Baby Luigi. He cries when he sees the ghost of the Cobalt Star which is really the Elder Princess Shroob's spirit or ghost.
Evil Twin: Princess Shroob has a very strikingly similar outfit to Princess Peach. Also, Elder Princess Shroob seems far more savage in comparision to her sister.
Heck, the Shroob Kingdom seem to have counterparts of everyone in the Mushroom Kingdom. From Wigglers (Swiggler), to Yoshi (Yoob), to possibly Mario and Luigi themselves (Elder Shrooboid and Junior Shrooboid, respectively).
Exotic Entree: When baby Bowser is choking on stolen cookies, he is offered milk from Kamek, who insists it's from an evil cow.
The Faceless: Princess Shroob's face is not actually seen up until the point you enter the Koopaseum. Later, when she poses as Peach, a Shroob mushroom on her face is part of her disguise. With a wig, dress, and gloves, she looks exactly like Peach.
The destroyed Toad Town section of the game is immediately followed by the celestial peace of Star Hill.
When you first arrive in the past, the first area has a bouncy, lively rendition of the original Mario theme, but then you get to Hollijolli village, you have a sad version of "Jingle Bells".
My Future Self and Me: The gameplay hook for this game has the adult and infant Mario Bros. teaming up inside and outside battle.
Having both generations of Bros. together in battle lets you use attacks and Bros. Items in new ways, such as double jumps and fireballs that can target air enemies.
Nice Job Breaking It, Hero: Assembling the Cobalt Star, which takes 95% of the game, undoes everything Peach sought to avoid from the start of the game and unleashes the Elder Princess Shroob
Nice Job Fixing It, Villain: Princess Shroob's plan to take over the modern day Mushroom Kingdom while disguised as Peach would have probably been much more successful had Bowser not shown up and attempted to kidnap her, hurling them both back into the past in the ensuing chaos.
Point of No Return: Used and subverted : The fight against the Shroob Mothership is the point of no return, but Stuffwell warns you about it. Even better, after you pass the PONR, he gives you the possibility to go back to before the fight, asking you every time you hit a save point.
Post-Climax Confrontation/Post Final Boss: When Mario, Luigi, and the babies return to the present after defeating the Elder Shroob Princess, what's left of the her possesses Bowser (turning him into "Shrowser") for one last battle.
Scrödinger's Plumber: The Copy Flower makes practically an infinite number of clones of the bros. The one that messes up is the real one, so theoretically all of the Bros. that attack are Real and Fake at the same time(until you mess up, anyways)
Sealed Evil In A Six Pack: The protagonists spend the game collecting several shards of the Cobalt Star. Only when they collect and put the pieces together do they realize that the star actually contained the leader of the invading aliens who was trapped inside by Princess Peach, and that by putting the star together, they release her.
Timey Wimey Ball: It's probably best not to analyze this at all, as the time travel doesn't even try to make sense. There's a great timey-wimey scene that combines Meanwhile, in the Future with Ripple-Effect-Proof Memory — The Bros give younger Professor E. Gadd a Eureka Moment, and we cut to older Professor Gadd, who is having the idea at the same time, and is aware that it's because his memory is changing.
Underground Monkey: Many enemies are merely Shroob versions of standard Mario enemies. One good example is the Tanoomba, which gets a Shroob variant in the final dungeon.
What Happened to the Mouse?: Tons, The captured toads are never rescued, Yoob is literally left hanging, Junior Shrooboid just vanishes, Toadiko and Toadbert are never seen returning to the present, and the remaining Shroob forces are not accounted for. The last two, however, are addressed in the sequel. Toadbert at least made it back, and The Koopa-Troop slaughtered the shroob army and froze the survivors.