->''[[MultipleEndings But this is not a true ending!]] [[GuideDangIt Come here with your friends!]]''
-->-- ''VideoGame/BubbleBobble''

An aspect of the game in which one can get additional content or powerups by interacting with other people who own the game. And then there are the games that can ''only'' be played in Multi-Player.

Supremely awesome if one actually has ''access'' to friends who own the game; a FanConvention is a great place to do this. Modern consoles and handhelds are able to go online, so many games with these features also include anonymous internet matchmaking.

Supremely annoying if no access is possible due to location, you're replaying an older game that isn't in sync with whatever is popular these days, or you simply don't have any friends. In this case, you might have to buy multiple copies and play with family, give them as gifts, word-of-mouth advertise to get your friends to buy the game, or just play entirely on your own on both copies. (This can make some Socialization Bonuses seem more like RevenueEnhancingDevices.)

Compare OneGameForThePriceOfTwo and PlayerDataSharing, where some games require this for HundredPercentCompletion. Contrast MultiplayerDifficultySpike, in which playing with other players makes the game more difficult. For bonuses gained from socializing with fictional characters, see LevelUpAtIntimacy5.
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!!Examples:
* If you GottaCatchThemAll in ''Franchise/{{Pokemon}}'', you must engage in this, since not only are the Pokémon [[OneGameForThePriceOfTwo divided up between games]], but every generation has some Pokémon which only evolve when traded. Also, Pokémon obtained in trades gain experience faster than Pokémon caught in your own game.
** Another example from Pokémon is the original Mystery Gift, which rewarded players with random items (usually decorations).
** Then there's the Spiritomb event in ''VideoGame/PokemonDiamondAndPearl'' and Platinum, where you must talk to 32 other players in the Underground to trigger Spiritomb's appearance so you can capture it. And [=NPCs=] don't count.
** ''VideoGame/PokemonBlack2AndWhite2'' add the "key system", which allows players to unlock special features in other players' games, including the different difficulty levels and the ability to swap out certain areas of the game world. Notably, the extra difficulty levels (one easier than normal, one harder) can ordinarily only be unlocked if you have beaten the game; the only way to play through the main story on the other difficulties is to have someone who's already beaten the game unlock it for you.
** Each campaign can catch only one starter Pokémon without trading. This, combined with the [[SaveGameLimits limit]] of one campaign per Game Card, leads to destructive behavior in "[[http://notalwaysright.com/gotta-catch-his-son/24609 Gotta Catch His Son]]" on ''Website/NotAlwaysRight''.
** The socialization aspect of the series is most likely the reason why the main Pokemon series will never be done on a console; because the series relies heavily on meeting new players while on the go in order to trade and battle with them, plus it is also done in-universe, Nintendo assumes people will go out and find other trainers to battle and trade with. It's fine and dandy if you live in a heavily populated area but if you live in a quiet town, you're boned. In other words, you have to go out, explore, and meet new people in order to get more Pokemon, just like you do in the actual games.
** Somewhat eased with the introduction of GTS in ''VideoGame/PokemonDiamondAndPearl''. The Global Trade Station is ''supposed'' to make finding Pokemon easier by letting you trade for them online, putting up your own Mons in exchange for another. The only problem with this is the enduring fad of people putting up ridiculously basic Pokemon for Legendaries, leading to a bit of a fan meme- "Did you find anyone who had a [[ComMons Seedot]]?" "Yeah but all five of them want Lv9 [[OlympusMons Mews]] for them." Not helping is the existence of a glitch that allows players to duplicate their Pokemon using the GTS, which usually leads to people asking for ''impossible'' trades (so that their Pokemon can't get traded away in the middle of doing the glitch).
* Many of the Pins in ''VideoGame/TheWorldEndsWithYou'' only "evolve" into other Pins if the points gained are from connecting with someone else with either a DS Wifi active, or another ''TWEWY'' game, also searching for other players. In theory, one can hope for random "alien" encounters for socialization points, but this would take days in real time to evolve a single Pin this way, days spent not actually playing the game. While the game was still new, it was very easy to get Mingle points in Japan due to the higher population density, but in other countries, even in major cities, finding other players without coordinating some sort of meetup is nearly impossible; the only sure-fire way to get 10 passes was to attend comic conventions; nowadays since the game is over five years old, it's unlikely you'll find players to Mingle with now.
** In fairness, these points are weighted; in absolute numeric value they do nothing extra to level up the pin, but pins evolve based on what type of point comprises the majority of experience on the pin, and the socialization points counted for nine times as much as battle points.
** There are also several Gatito pins that only work if you have a complete set consisting of several pins. There are only two ways to complete the set: one is through [[RandomlyDrops incredibly low-percentage]] drops on Ultimate difficulty from BonusBoss fights, and the other is to buy them from stores... which only sell one pin per set ''per DS''. To buy the others, you have to play the game on other DS units.
* You simply ''cannot play'' The ''VideoGame/TheLegendOfZeldaFourSwords'' part of [=GBA=] ''VideoGame/TheLegendOfZeldaALinkToThePast[=/=]The Four Swords'' combination if you don't have other players who have their own GBA systems and copies of the game. Without them, you can at least watch the opening cutscene...
** Hilariously, two aspects of ''A Link to the Past'' cannot be ''accessed'' without first completing ''The Four Swords''. The first being the quizlike series of [[FetchQuest Fed-ex quests]] to receive an upgraded spin attack, and the second being [[BonusDungeon the Palace of the Four Swords]]. [[spoiler:Well, at least said palace was accessible and completable by exploiting a glitch that was present in the original SNES ''[=ALttP=]'', but don't tell Nintendo that.]]
** In celebration of the 25th anniversary of the Zelda franchise, Nintendo made the Four Swords game for [=DSiWare=] to be played with one person if there are no other players to play with. Playing in single player has a 2nd Link tag along who doesn't do anything but follow you and help you set up for executions requiring the actions of multiple players at once and both Links can be swapped should you need to split them up. However, there are several bonus levels that qualify as a BrutalBonusLevel because of the huge amount of enemies that are thrown your way and some are also EliteMooks, thus you need friends to help you stay alive.
* Many powerful chips in the ''MegaManBattleNetwork'' series are only possible via this method.
** In the ''VideoGame/MegaManStarForce'' games, you can collar extra Link Power by forming "[=BrotherBands=]" with other people who own the game in question, allowing you to equip more and beefier abilities and qualify for certain sidequests and items earlier than you normally would. Plus, well, [[OneGameForThePriceOfTwo "Hey, guy with Zerker x Ninja! I've got Zerker x Saurian. How 'bout we both get ourselves access to Tribe King?"]]
*** ''Star Force 2'' had an inversion in the Indie Fragments - using them gives you a variant on your old SuperMode that enhances sword attacks and can never be removed by enemy action...but [[PowerAtAPrice they take up your six Brother slots]], so if you have any non-storyline friends, you have to reject them to access Mega Man Rogue. (As an interesting side effect, it also becomes impossible to use the ability that lets you auto-enter SuperMode at the beginning of combat.)
* In ''VideoGame/FinalFantasyIII DS'', sending a sufficient number of mail messages to other owners of that game via the NintendoDS e-mailer unlocks the LethalJokeCharacter class and extra dungeon.
* This is what makes ''RockBand'' fun, as the main meat of the game (the Band World Tour) is only accessible in multiplayer. The more players, the better.
** Though if you are a decent singer (or settle for the lower rewards of lower difficulties), you can play any instrument while singing yourself. It's hard, but also ''epic awesome''.
** ''Rock Band 2'' reduces the impact of this trope in some ways (offering single-player career-like "challenges"), but improves it in other ways (said challenges are easier with a band to help correct your screwups).
* ''VideoGame/MortalKombatShaolinMonks'' has elements of this, in that some of the secrets are only accessible in co-op mode. None of them are ultimately that essential, though, and generally just consist of artwork.
* Some ''VideoGame/TeamFortress2'' achievements (particularly the Medic achievements required for the [[InfinityPlusOneSword Infinity Plus One Bonesaw]], before the requirements got lowered) require the use of {{Steam}}'s not-entirely-intuitive Friends list.
** Then there's the achievement for playing a game with seven people on your Steam Friends list. Too bad if you don't have seven Steam Friends you can get together, or even seven friends.
** Similarly, many of the Medic achievements needed another player to do something totally counter-intuitive (like a heavy punching people while Ubered), that they effectively required a cooperative partner to help you boost them.
* While not required, all ''{{Creatures}}'' games allowed players to save their Creatures as discrete files and give them to friends (or put them in stasis for six generations, or clone umpteen times), and ''Docking Station'' introduced the ability to send them over the internet within the game. Additionally, ''Creatures 2'' was somewhat backward-compatible, allowing creatures from the first game to be converted for the second.
* ''[[GaiaOnline zOMG!]]'' places a heavy emphasis on socialization. The game is designed in such a way that if you don't find some people to crew with, you're not going to progress very far. In fact, the path to the FinalBoss for the launch chapter requires synchronization between at least 3 players to open. In addition, your character gets bonuses by interacting with other players, called [[TheForce G'hi]] bonuses. You can fill your G'hi up by visiting Barton Town or joining a 6 person crew. But if your G'hi runs out, all of your bonuses will be gone until you can get more G'hi.
** Most {{MMORPG}}s, if not all, require players to work with others to complete the most challenging dungeons. ''VideoGame/WorldOfWarcraft'' and most [=MMORPGs=] with a crafting system make socialization necessary to access most crafted equipment by limiting the number of crafting skills a single character can take.
*** In the earlier days of ''VideoGame/FinalFantasyXI'', if you wanted nearly ''anything'' done, you needed a party. A major reason people left FFXI was simply because there were times where you could do ''nothing'' but wait for a group. Missions? Party. EXP(That didn't take forever)? Party. Quests? Party. Get unsellable gear from a BonusBoss? Party. The things you ''could'' do on your own were very limited, and most of them only possible at the cap of level 75. Nowadays, there's a lot more solo content, but most things still require more than one person to do.
**** It is apparently also still a problem if you chose to become an undesirable class. Are you a [[GameBreaker Red Mage]]? People will practically pay you to join their group. A [[JokeCharacter Puppeteer]]? You might as well give SE another dollar so you can start over.
* ''SuperSmashBros. Melee'' has Mewtwo, who is unlocked by playing Vs. mode for 20 hours. However, each vs. match counts for every human player involved, meaning that having more players unlocks Mewtwo faster.
** However, the 20 hour requirement can easily be bypassed by having the game being played in a timed match with the clock set to infinite. Leave the game running until the 20 hours have passed.
*** Or combine it with the above, plug in 4 controllers, leave it on for 5 hours.
** Super Smash Bros Brawl also has content that's either easier with extra players or only possible with such. Like the whole seperate set of events you can only play in co-op mode in Brawl. There are also levels and modes which are much easier (and arguably only possible for many people) in co-op, like the lengthier Multi Man Brawl modes (15 Minute Brawl is a good example of something that's much more doable in a team) or much of the Subspace Emissary on harder difficulty levels.
* Crafting in KingdomOfLoathing. The best foods are Hi Meins. Creating them is a three-step process, each step requiring the level 15 skill of a different class. The last step actually requires that you ''be'' a Mysticality class, so [[NewGamePlus skill perms]] won't help you there.
** There are also [[BonusDungeon Hobopolis and the Slimetube]] which require you to be in a Clan in order to play in them, and while it is possible to get the best rewards in Slimetube [[AvertedTrope by yourself]] You'll want a team of 6 people in order to find the best items in Hobopolis.
** And the mall, as well as trading in general. Honestly, despite the label of an MMORPG, Kingdom Of Loathing is a single player game with a few socialization bonuses.
* In ''VideoGame/{{Boktai}}'', the Azure Tower contain the strongest of each type of Gun Frame, but you can only get ones that you have the proper seal for, with there being seven total (for each of the elements in the game). With one cartridge you're only given the Dark seal in the NewGamePlus, the Sol seal after growing the Solar Tree (which takes a couple of hours in direct sunlight), and one random one (out of four) when you enter the tower for the first time. The rest you need to get by linking to someone else. What's worse, only the random one is transferable through this method - even if the other person has all seven, you'll only get one - and if it's the same type, well...
* Played straight ''and'' averted in ''VideoGame/LittleBigPlanet''. "Acing" a level (clearing it without dying once) is virtually impossible in multiplayer, so certain rewards are essentially locked to solo play. Meanwhile, many segments have multiplayer areas whose rewards require multiple people to solve a puzzle. Up until the Wilderness, this is usually two players - after that, many of them require a team of four. (Thankfully, that always-handy "Play Online" button and good luck with headsets makes this much easier here than in some examples...)
* Some of the sidequests and MultipleEndings in ''VideoGame/ResidentEvilOutbreak'' would only work on Multiplayer because of triggered events. [[LostForever Too bad, because there's no multiplayer for those games any more.]]
* You could play the original ''VideoGame/FinalFantasyCrystalChronicles'' by yourself, with a Moogle carrying the myrrh bucket for you, but it was far more satisfying to play alongside friends so you could argue over [[ScrappyMechanic who had to carry the bucket]] through the level. However, doing this required as many {{Game Boy Advance}}s and {{GameCube}}-GBA link cables as there were players.
* ''VideoGame/KingdomHearts358DaysOver2'' has Mission Mode, which lets you replay completed missions from Story Mode in order to gain Mission Crowns, which can be redeemed for bonuses from the Moogle. In the Japanese version, Mission Mode can only be accessed in multiplayer format, while the American version is more liberal, enabling you to play Mission Mode either with friends or by yourself, though the enemy stats are boosted regardless, making it NintendoHard. Still, you do get EXP even if you fail, so you can keep trying until you're strong enough to win. Playing Mission Mode is the only way to obtain the maximum number of inventory slots available, and it is the only feasible way to reach Lv. 100, as the gaps between levels are so wide at the end of the game that playing without level multipliers (some of which are only available in Mission Mode) would qualify as a LowLevelRun.
* The Camraderie stat in ''VideoGame/MetalGearSolidPeaceWalker''.
* ''VideoGame/RuneScape'' has (at last count) two quests in which you must team up with other players. The first of these quests assigns you to a gang, so you have to find someone else who is in the opposite gang. You only really need the other player for a few minutes though.
* ''VideoGame/Left4Dead'''s AI bots don't have the necessary perception to leave a partner untreated when temporary health is a better option, they can't use any kind of grenades, and they frequently interrupt your shooting by giving you pills or adrenaline at the most inopportune times, and in [[VideoGame/Left4Dead2 the second game]], they don't collect special ammo or use melee weapons either. They aren't ''completely'' [[ArtificialStupidity incompetent]], being able to shoot very accurately and very rarely getting lost, but they're vastly inferior even to somewhat inexperienced human players.
* In ''VideoGame/NewSuperMarioBrosWii'' and ''VideoGame/DonkeyKongCountryReturns'', if all but one player die, they can be resurrected by the one still standing. Makes beating some levels [[GameBreaker MUCH easier]], [[NintendoHard and maybe even necessary]].
* With the [=SpotPass=] and [=StreetPass=] features of the Nintendo3DS, Nintendo has integrated this trope into a system itself rather than a game.
** ''VideoGame/MarioKart 7'' uses the [=StreetPass=] feature to unlock the Gold Glider part. The more people you connect with the feature, the less coins you need to unlock the part. Don't live in a place where lots of people own a copy of the game and use the [=StreetPass=] feature? Have fun grinding for 10,000 coins to get that Gold Gilder.
** ''VideoGame/AnimalCrossing: New Leaf'' has the Happy Home Academy Showcase, where [=StreetPassing=] other ''New Leaf'' players allows you to access a model of their home, where you can buy their furniture (albeit at a pretty large markup). While some items ({{DLC}}, seasonal items, fortune cookie rewards, etc.) can't be bought from the HHA Showcase, if you [=StreetPass=] someone who has rare furniture, you can buy it from them much more quickly/more easily than you can obtain it normally. Assuming you have the Bells to pay for it.
* Many Xbox 360 games from the first two years had achievements that required local co-op (before Xbox Live play became basically universal), notably ''VideoGame/GuitarHero II'', in which 1/5th of the game's achievements were based on local co-op.
* ''VideoGame/GuitarHero III'' had the same co-op of Guitar Hero II (second player plays bass/rhythm guitar). That, alone, isn't bad at all. What makes it bad is that it introduces co-op career mode [[CaptainObvious that must be played with two players]], and said mode includes six songs that aren't unlocked in the course of single-player mode or the unlock shop (though, mercifully, a cheat code is available that unlocks everything).
* ''{{Pac-Man}} Vs.'', originally bundled with ''Pac-Man World 2'', is only playable in multiplayer and requires one GBA and the special cable.
* ''VideoGame/MetroidPrime 3: Corruption'' includes a feature wherein you can use "credit" icons obtained in the game to buy extra stuff. The green credits, however, can't be obtained on your own... you have to have someone on your Wii's friend list send you "Friend Vouchers" that ''they'' got on ''their'' game (the vouchers are totally useless on your own game.) Players also have to make sure their friend has the same version of the game, otherwise the feature won't work.
* The PSP remake of ''VideoGame/FinalFantasyTactics'' requires you to link up with another player to do the multiplayer battles that give out some of the uber equipment that could only be gotten via theft in the US version (not the Japanese version) of original.
* ''TraumaCenter: New Blood'' can be completed entirely in singleplayer, but having a competent partner in co-op makes some operations much easier: not only do you have a second pair of hands, but you can use both Healing Touches in the same operation.
* ''TestDrive Unlimited 2'', you level up your social level by challenging players, participating in chases or co-driving and many other things. Playing Poker and Roulette with other players in the casino levels up a seperate social level on Casino Island.
* In ''VideoGame/MarvelAvengersAlliance'', the most reliable way to acquire most resources is to get them from other people who play the game.
* ''VideoGame/AnimalCrossing'' starts each town with only one out of five kinds of fruit on the trees; the easiest way to get other kinds of fruit is to visit another player's town. You can also only find certain items in a furniture set at a store in another player's town. Furthermore, the first two ''AC'' games required someone from another town to buy or sell something in your town's Nookway store in order to get Nookington's. Thankfully, ''AC: City Folk'' did away with this, probably because of the game-imposed limitation of one town per console (unlike the first ''AC'') and the practical limitation of one console per family.
* Two less-well-known games for the GBA, ''Videogame/SpyroOrangeTheCortexConspiracy'' and ''VideoGame/CrashBandicootPurpleRiptosRampage'' (otherwise known as ''Spyro Fusion'' and ''Crash Fusion''), require you to trade or play multiplayer games with a player with the other cartridge if you want all the cards.
* The the options available for the LastSecondEndingChoice in ''VideoGame/MassEffect3'' depend on your Effective Military Strength score, which is calculated by multiplying the total of your War Assets (collected in single-player) by your Readiness Rating. Said rating is set to 50% by default and can only be increased by completing multiplayer missions or [[RevenueEnhancingDevices playing tie-in games for other consoles]]. Before the Extended Cut came out, this effectively meant that unless you played the multiplayer, you could never unlock the best (that is, [[PyrrhicVictory least destructive]]) endings. With the Extended Cut installed, however, the EMS requirements for the best endings are lowered, allowing a player to unlock them entirely through single-player (though playing multiplayer makes it much easier to do so).
* While you can complete any class storyline in ''VideoGame/StarWarsTheOldRepublic'' on your own, with just the AI companions, you get "Social Points" for beating quests (and particularly picking matching dialogue responses) with other players, which, in turn, unlocks unique equipment. Not to mention that playing in a party increases the likelihood of rare loot.
* It is possible to complete ''VideoGame/{{Journey}}'' on your own but the entire game is obviously designed to be played with an anonymous Companion over the net. Sticking together also makes some stages easier, since you can [[ThePowerOfFriendship endlessly replenish each other's energy--but not your own]].
* Achievements that require another player to help you usually become ThatOneAchievement when the game's popularity dies down or the game servers are shut off.
* Many {{Facebook}} games are ''downright impossible'' to play alone, because the game refuses to give you items which are required for certain quests. Instead, you must beg for them from your friends, which requires either alienating your real-life friends with endless "Send me TwentyBearAsses!" requests or making "friends" with perfect strangers who happen to already play the game (and hope that they do not quit playing or run amok on your wall).
* ''VideoGame/SinAndPunishmentStarSuccessor'' with a second player is ''significantly'' easier. Player 2 only shows up as a cursor rather than a second character, which means they're in no danger of taking harm (though this does mean [[WeCannotGoOnWithoutYou Player 1's defeat means a game over for both players]]). The game averts MultiplayerDifficultySpike, so it's perfectly possible for Player 2 to take breaks and still complete the game or for Player 1 to complete 2P mode without the second controller being touched ''at all'' (though that's missing the whole point of the mode).
* ''VideoGame/{{Ingress}}'' requires players to collaborate if they want to do anything meaningful.
** A Portal can only be powered to Level 5 at most by one player, as players have limits on how many resonators of each level they can put on one portal; at Level 8, a player can finally put down level 8 Resonators but can only put down one per portal, so eight Level 8 players are needed to bring a portal up to level 8.
** A Portal can only accept two mods from each player. To fill all four mod slots, at least two players are required.
** Links and Control Fields can be done by a single player, given the proper keys. But it's much easier and faster if multiple players work to assemble the links and fields, as the drop rate for a key drops dramatically if the player already has one or more of that key, and as fields and links get longer, more players are needed to reduce the travel time and distance for each player.
* ''VideoGame/OneWayHeroics'' features online functionality, allowing you to "see" the progress of other players in the [[ProceduralGeneration same seeded world]], warn others of dangers in that world should you succumb to a particularly nasty fight, share random seeds to get other players into interesting worlds you discover, check leaderboards to compete for skill and bragging rights, and even share your exploits and advice on Twitter! Meeting Ghosts of players who fell in these worlds will also give your extra experience and some helpful items.
* ''Roleplay/DestroyTheGodmodder'': You get charges maxed out far faster if you can convince the other players to help you, especially if they're frequent posters.
* ''VideoGame/{{Spelunky}} HD'' rewards you with an achievement if you complete the game with at least two alive players.
* ''VideoGame/{{Minecraft}}'' has an achievement gained by tossing a diamond towards another player. Thankfully, this can be done in single-player mode as well, since tossing a diamond at any mob that can hold items also counts.
* ''VideoGame/MonsterHunter'' quests are easier to do with multiple players, ideally the full four-player party. There is absolutely no penalty for completing that Urgent Quest you need to raise you HR with a full party, and on top of that, monster carves are localized, thereby averting LootDrama.
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