On a night outside of Princess Peach's Castle, Mario and his friends came to watch the Mini Stars in the sky. As Mario peers though the telescope, he notices that the stars suddenly begin to get sucked through a vortex. It is then revealed to be Bowser and Bowser Jr. on a spacecraft, who are using a vacuum-like machine to suck the stars in the sky. Upon witnessing this, Mario and the gang set out to defeat them and save the Mini Stars. After they start their journey, Shy Guy and Kamek are seen coming out of the woods and start following them, as part of Bowser's plan.
A new form of gameplay was introduced in this game, retiring the old format seen in the prior eight titles: players move all at once through the board in a vehicle (a car in Toad Road, a magic carpet in Boo's Horror Castle and a legged machine in Bob-omb Factory, for example). Instead of trying to collect coins to buy stars, players receive Mini Stars if they pass by them. New minigame types are be introduced in the Mario Party series, one example being 2 vs. 1 minigames against Bowser Jr. Players must try to avoid Mini Ztars, which deduct their current amount of Mini Stars. In this game, the minigames don't appear after every four turns, but only when a player ends up on any of the spaces that triggers a minigame. Unlike previous Mario Party games, where often only the winner mattered, all minigames are ranked from first to last place and generally all players receive Mini Stars although the winning player earns the most.
Mario Party 9 has 5 modes in all. They are:
- Party Mode
- Solo Mode
- Minigame Mode
- Extras Mode
Blue Toad is the host of Minigame Mode.
- Free Play: In the Free Play mode, the players can play unlocked minigames freely.
- Step It Up: In this mode, players must win minigames to climb stairs. The player who reaches on the top first wins.
- Garden Battle: The players must get puzzle pieces to fill their gardens winning minigames. The player who completes it first wins.
- Choice Challenge: All the players choose three minigames from a group of five that they would like to play. These minigames are played and the player who get more points wins.
- High Rollers: In this mode, the players must win minigames to roll a Dice Block to pass through a panel floor. The player who gets 500 points first wins.
- Time Attack: The player must try to win 10 minigames as fast as he or she can.
- Boss Rush: The players must defeat a series of bosses to win, similar to Boss Bash in Mario Party DS.
The Museum is where the player can return constellations back into the sky, buy boards or vehicles, or watch the credits. Once they return a constellation into the sky, the player can go back and see it in the sky.
- Shy Guy
There are a total of 14 boss battles in Mario Party 9.
- Toad Road: Lakitu and Wiggler.
- Bob-omb Factory: Whomp and King Bob-omb.
- Boo's Horror Castle: Dry Bones and King Boo.
- Blooper Beach: Cheep-Cheep and Blooper.
- Magma Mine: Spike and Chain Chomp.
- Bowser Station: Bowser Jr. and Bowser.
- DK's Jungle Ruins: Diddy Kong and Donkey Kong.
- Yello Toad
- Green Toad
- Blue Toad
- Dry Bones
- Bullet Bills
- Piranha Plants
- Huckit Crabs
- Monty Moles
- Hammer Bros.
- Fire Bros.
- Buzzy Beetles
- Mr. Blizzards
- Scaredy Rat
- Toad Road: Toad Road is the first stage in Mario Party 9, and thus it is a simple stage with almost no gimmicks. It is designed after New Super Mario Bros. Wii, and features lush grassland, flowers and windmills. A broken bridge prevents players from moving forward, and if the Energy Meter does not add up, the players will fall to a lower route with Mini Ztar Spaces.
- Bob-omb Factory: Bomb-omb Factory is the second stage in Mario Party 9. Players move around a room full of conveyor belts, and if the Captain lands on an Event Space, the conveyor belts will move the group of players around, including spaces. Bob-ombs will occasionally intrude a player's game. Whatever Captain happens to stop the Bob-omb's countdown, he or she will lose half of their Mini Stars.
- Boo's Horror Castle: Boo's Horror Castle is the third stage in Mario Party 9. Players move throughout a big castle, which is haunted with Boos. After a Captain passes a Boo Portrait, a Boo will come out of it. If a Captain is caught by a Boo, he or she will lose half of their Mini Stars. Boos will follow the players until they leave the current hallway, or enter an alternative room with lights. Before the players can face the boss, the Captain must roll a number higher than the specified numeral on the gate.
- Blooper Beach: Blooper Beach is the fourth stage in Mario Party 9. Players travel along an ocean to reach the end goal. If a Captain passes by a Dolphin, he or she will get five Mini Stars. However, Sushis will take away half of the Captain's Mini Stars. Huckit Crabs will turn all the Mini Stars on the stage into Mini Ztars if a Captain lands on an Event Space.
- Magma Mine: Magma Mine is the fifth stage in Mario Party 9. Players must escape rising magma that rises two spaces each turn, and is increased further if a Captain lands on a Magma Space. If a Captain happens to get hit by magma, he or she will lose half their Mini Stars.
- Bowser Station: Bowser Station is the sixth stage in Mario Party 9. Bowser Jr. and Bowser are the stage's mid-boss and boss, respectively. A Jackpot Machine here will increase in Mini Stars if a Captain lands on a Jackpot+ Space. If the Jackpot Machine happens to have 20 or more Mini Stars, a Jackpot Minigame will commence. Oddly enough, this stage has no actual hazards.
- DK's Jungle Ruins: DK's Jungle Ruins is the seventh stage in Mario Party 9, which can be purchased from the Museum for 500 Party Points. Instead of collecting Mini Stars, players collect Bananas while avoiding Z-Bananas. Players make two laps around the stage before completing the whole set. Bananas and Z-Bananas are placed on each space rather than inbetween. The actual Boss Minigames in this stage are done in form of a bonus minigame to collect Bananas. These games are supported by Diddy Kong and Donkey Kong.
- Logger Heads is a pun on loggerhead.
- Launch Break is a pun on lunch break.
- Skyjinks is a pun on hijinx and sky.
- Skipping Class is a play on skipping class.
- Mecha Choice is a pun on make a choice.
- Jigsaw Jumble is a pun on jigsaw puzzle.
- Upward Mobility is a wordplay on the term downward mobility.
- Ring Leader is a play on ringleader, a person who leads others.
- Smash Compactor is a pun on trash compactor.
- Card Smarts is a pun on the game show Card Sharks.
- Toad and Go Seek is a portmanteau on the game, hide and seek.
- Twist Ending is a play on the term, twist ending.
- Manor of Escape is a pun on manner of escape.
- Flinger Painting is a pun on finger painting.
- Plunder Ground is a pun on plunder and underground.
- Billistics is a pun on ballistsics and Bullet Bill.
- Thwomper Room is a pun on Romper Room and Thwomps.
- Urn It is a pun on earn it.
- Pizza Me, Mario is a pun on Mario's famous phrase, it's-a me, Mario!
- Line in the Sand is based on the phrase, "line in the sand".
- Block and Roll is a pun on the music style, Rock 'n' Roll.
- Spike-n-Span is a pun on spick and span.
- Mob Sleds is a pun on bobsleds.
Bowser Jr. MinigamesEdit
- Cage Match is a play on cage match.
- Sand Trap is a play on sand trap.
- Pedal to the Paddle is a pun on Pedal to the Metal.
- Dice Block
- Slow Dice Block
- 0-1 Dice Block
- 1-2-3 Dice Block
- 4-5-6 Dice Block
- 1-10 Dice Block
- Mini Stars
- Mini Ztars
- Green Shells
- Banana Bunches
When a player lands on a space, something is going to happen. Each space has its own effect, but like in previous Mario Party games, every space is colour-coded. The colour of the space will tell what kind of event is going to happen.
- Normal Spaces
- Blue Spaces - Triggers good events and will help the player who landed on it.
- Red Spaces - Trigger bad events and will hinder the player who landed on it.
- Green Spaces - Lets one or all players attend an event.
- Yellow Spaces - Trigger minigames when landed on.
- White Spaces - Adds Mini Stars to the Jackpot on the Bowser Station board.
- Star Spaces
- Mini Star Spaces - Gives Mini Stars to the player who landed on it.
- Mini Ztar Spaces - Takes Mini Stars from the player who landed on it.
If the player moves over a space with a half cross on it, he or she automatically stops at that space and an event starts. The only two spaces of this kind are the Boss Space and the Captain Event Space.
Because of the new rules in Mario Party 9, there are many new spaces introduced. Some old spaces returned as well, but some have a different effect (for example the Blue Space). With a total of 21 different kind of spaces, Mario Party 9 has the most spaces of the series. Also for the first time in the Mario Party series, some spaces are unique for a specific board. Here is a list of all the spaces in the game, with what kind of effect they have.
- The players' cursors when selecting options and playing minigames are the hands of their characters, similar to the hands from the first four Mario Parties, and the Hedge Hocho minigame from Mario Party DS.
- This is the first Mario Party in the series not to be developed by Hudsen.
- This is the first Mario Party to not include Advice or Hints in the Minigame Rules Screen.
- This is also the first Mario Party game to not include a category of 2-vs-2-Minigames, and the first since Mario Party 2 to not include a category of Duel Minigames.