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 moves in battle have been replaced by Bros.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.
And I Must Scream: The fate of some Toads early in the game. Poor Toadiko and later Toadbert. Fortunately, they are revived at the end, but that doesn't make it any less scary.
Angry Fist-Shake: Luigi when Stuffwell lands on him after coming out of the first time hole. Also when Baby Mario snaps Luigi back to consciousness with a hammer whack to the head. This makes the babies cry causing Mario to get angry causing Luigi to then cry.
Big Damn Heroes: Kylie Koopa disguises herself as a Shroob and ambushes the ones ambushing the heroes on the mothership.
Big Eater: Baby Mario and Baby Luigi sure love Yoshi Cookies! So much that they eat all of Baby Bowser's ill-gotten gains. He isn't amused.
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.
Boss Remix: Bowser's battle theme remixes his leitmotif (which has two variations, for his young and adult selves).
Buried Alive: Petey Piranha does this to himself in his battle with him, although he only does it up to his oversized head.
Butt Monkey: While the Mario & Luigi series loves to poke fun at Luigi, this game ramps it Up to Eleven and makes him a veritable Cosmic Plaything. Luigi is not only an Iron Butt Monkey (he takes a huge amount of abuse in game from enemies, objects, and Baby Mario's Hammer), but suffers through the entire game as a No Respect Guy on top of it (a few of the Toads spend a not-insignificant amount of time suggesting Luigi leave the world-saving to Mario). And then there's the Stargate, who abuses Luigi so thoroughly that he got himself an entry under The Scrappy on this game's YMMV page.
Defanged Horrors: As a Mario Bros game, it clearly refrains from reveling in gruesome imagery, but there's some pretty horrific stuff going on that it glosses over. Take the Yoob, the giant Shroobified Yoshi that idly rampages across Yoshi's Island and consumes its inhabitants one by one. It doesn't actually eat them; located within its innards is a factory where the yoshis are converted, one by one, into Yoob eggs. Stuffwell and all four Mario Bros react with revulsion.
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.
Going for the Big Scoop: Kylie's personal goal in the game is to write a book about the Shroob Invasion. She succeeds.
Gold Colored Superiority: The game had a golden version of a early mook that appears halfway through the game. Although this one has lower health than the original, it compensates by having a higher defense and attack power.
Hopeless Boss Fight: When Mario and Luigi first encounter the Shroobs, they get drawn into a fight with three Shroob enemies. Each of these enemies has about 15 HP and a simple attack pattern... but there's also an UFO flying overhead with a countdown timer set to three turns (after which it'll take the bros out but good). At their current power level, it's barely possible to take down one of these guys in that time frame.
This segues into the Babies pulling off a Big Damn Heroes and using these same three Shroobs as a tutorial fight for Bros. Items.
I Hate Past Me: Bowser ends up meeting his past self in Thwomp Volcano, and the two of them proceed to fight over which one gets to use the place as an evil lair. They eventually decide to team up, but neither one ever figures out the other's identity.
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.
Schrödinger'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?"
That Liar Lies: The Star Gate accuses Luigi of this when Luigi was only given three options instead of 4: Everyone! The Star Gate is the true liar but it's all part of a test.
Theme Naming: All Shroob lifeforms and otherwise Shroobified enemies will be indicated not only by their appearance but by the structure of their names. Natural Shroobs tend to have the full word "Shroob" in their name, while Shroobified classic Mario Bros. enemies tend to have "S-", "Shr-", or "-oob" appended somewhere to their name.
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 and bosses are "Shroobified" versions of standard Mario enemies. One good example is the Tanoomba (itself an Underground Monkey of the basic Goomba), which gets a Shroob variant (the Tashrooba) in the final dungeon.
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.