The Subspace Emissary is the adventure mode of Brawl, and takes up a significant portion of the game. It is similar to a beat-em-up game, featuring playable characters fighting across multiple stages. Up to two players can play in the mode.
The Subspace Emissary involves multiple stages spread across an overworld map. A new stage is unlocked by clearing the one before. Due to the events of the storyline, some stages might be temporarily unavailable, such as the Midair Stadium, which remains unplayable until after you defeat the final boss. Each stage consists of basic platforming as well as unique enemies. Stages may also contain boss battles, and even fights similar to a normal Versus Mode match.
Each stage has a completion status that counts toward the overall completion percentage that is displayed on the overworld map. An uncleared stage will have a red icon. An incomplete stage will have a flag on its icon after clearing it, while a totally complete stage will have a rainbow crown icon. To reach total completion in a level, you must enter every door in the stage, break apart every gold item box and collect every openly displayed trophies within the stage.
Combat in the Subspace Emissary is similar to normal brawling, except you're now fighting multiple weak enemies instead of other players. All attacks are still available to you except Final Smashes, since Smash Balls don't spawn in the mode. Grabbing also works slightly differently. Enemies are released almost immediately after being grabbed, making it impossible to pummel, so you must input the direction of your throw instantly after grabbing.
While playing the mode, you will have a pre-designated amount of stocks represented by the icons in the vertical black bar at the left side of the screen. Getting KOed from attacks or from falling into bottomless pits will result in you losing a stock. When you respawn, you will switch to another character available for the level, unless there is a second player already using that character.
When playing with two players, the camera will only focus on the first player. Doors can also only be entered by the first player. The second player can get KOed by the stage if the first character moves too far away, but they have the option of pressing the button that will normally pause the game to "warp" to the first player's location.
Additionally, there are small but noticeable balance changes specific to the mode. The speed of each character has been adjusted, so that faster characters move slower and vice versa. Gravity is stronger, and every character jumps higher and falls faster, with Peach having a noticeable increase in height with her double jump. Characters can also swim for much longer, while special moves that can only grab opponents will do damage instead.
The Subspace Emissary is the only mode where stickers serve a practical function. Players can attach stickers to individual character trophies on the stage select screen, giving that character various buffs. You can put as many stickers on a trophy as you want, but they cannot overlap and similar effects do not stack. Note that removing a sticker will permanently discard it from your collection, but you can obtain it again.