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 previously appeared in the Yoshi's Island games as well as some of the sports game spinoffs. 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 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 moves in battle have been replaced by Bros.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.
Dummied Out: The "Scoot-Bloop" enemy, which would have originally appeared in the Vim Factory.
Easily Thwarted Alien Invasion: An entire alien army is bested by four plumbers, and two of them were infants! Hell, one of said infants' tears were all it takes to undo the damage they've done !
Evil Counterpart: The Shroobs often act like this to the Toads, even having an enemy ingame known as the Shroobsworth. The Shroobs also have evil counterparts to other Mario characters, such as Princess Shroob to Princess Peach, Yoob to Yoshi, and the Shrooboids to Mario and Luigi (note that the elder one is red and the junior green).
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.
Toadsworth was also flanderized in this game: In his debut game, Super Mario Sunshine, he was obviously worried about Peach, but remained somewhat level-headed about it. Come this game, he's paranoid to the point of believing his past self to be a criminal who turned Peach into a baby to kidnap her, and doesn't trust him too much even after he learns the truth. Though on the plus side, it does give him more of a personality than he had in Sunshine.
Ironic Nursery Tune: Hollijolli Village's background music is based around a depressing-sounding version of Jingle Bells.
Killed to Uphold the Masquerade: Not exactly "killed", but the Shroobs have a frustratingly chronic habit of invoking this. Whenever a character tries to explain a single word about the Cobalt Star and what it contains, he or she will be shroobified before getting to finish. The only exception is Princess Peach herself, who instead gets interrupted constantly until Baby Bowser fully reassembles the star.
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, where 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.
She does look perfectly happy in her official artwork, though.
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 Final Boss: When Mario, Luigi, and the babies return to the present after defeating the Elder Shroob Princess, what's left of 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.
Like in the prequel, Bowser causes everyone a headache by choosing to kidnap Princess Peach at the worst possible moment. This time it turns out to be a good thing however, as "Princess Peach" was actually a disguised Princess Shroob.
Baby Bowser plays this trope in a much worse fashion, suddenly appearing in Shroob Castle to assemble the Cobalt Star after the Bros. were told not to. Cue Final Boss.
Speaking Simlish: Everyone. The Bros' version sounds vaguely Italian, with a few recognizable words; the Shroobs are entirely tonal.
Sucking-In Lines: Serves as a tell for the Snoozorb enemy. If the lines are spinning, the laser they shoot will go overhead, meaning that you shouldn't jump.
Sugar Apocalypse: What happens to Toad Town and Hollijolli Village. The latter is probably more unsettling, as it happens right as Mario and Luigi are watching, and right during Christmas.
Take That: The Brainwashed Hammer Bros. speak entirely in 1337-speak in the English release. After they're freed from the mind control, one of them remarks "Seriously, who talks like that?"
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.
A less apparent example occurs with Princess Shroob disguising herself as Peach to take over the Mushroom Kingdom of the present. It is quite clear that the Mario Bros. and their baby counterparts are the only people in the Kingdom capable of fighting off the Shroob invasion, so every time they return to the present before the Shroobs are defeated, they are leaving the past Kingdom completely vulnerable to Shroob takeover. Princess Shroob shouldn't need to go to the present because the present Peach's Castle should be Shroob Castle without anyone to fight against it over the timeskip.
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.
Vile Villain, Saccharine Show: The Shroobs. Murderous psychopaths who attack innocent civilians, try to use Toad souls/life force for UFO fuel and turn the entire world into Mordor. And they attack Hollijolli Village right on Christmas Eve. And additionally seem quite fine with brutally trying to murder the baby forms of Mario and Luigi (and any other characters). They're why this is seen as one of the darkest games in the franchise.
Villainous Breakdown: Given what a short temper Princess Shroob has, it's not surprising that she eventually goes berserk following the destruction of her Shroob mothership...
Weaksauce Weakness: How to dispose of evil toxic fungal life-sucking alien Bio-Weapons? Cry on them.
What Happened to the Mouse?: Tons. The captured toads, though revived at the end, are never directly 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.
Where It All Began: The endgame takes place in Shroob Castle, which is the Shroobified version of Peach's Castle, the first area you explore. The Post Final Boss fight with Shrowser also takes place in Peach's throne room, where you first fought Baby Bowser and Junior Shrooboid at the very beginning.