In the Demo Version, players assume the role of the promotional player character Orlando (Japanese: オメガ Omega), who shares the appearance of Brendan. The Demo is set in Mossdeep City and various Mirage spots. During the first adventure, the player assists Steven in stopping Team Aqua or Team Magma's plot to catch a Pokémon that can Mega Evolve. Subsequent adventures feature the player helping Steven complete other miscellaneous missions, including a couple of special missions that only appear rarely or when the right conditions are met. On every fifth adventure, the player will face an Expert who will challenge the player to a battle.
During gameplay, the player can only access the Pokémon and Bag menu options, and when the menu is closed, the bottom screen shows TV Mauville's Pokémon News on the BuzzNav. The Demo Version only saves after the player has completed an adventure, immediately before it returns to the title screen.
As the player clears more adventures, more non-playable characters (NPCs) will fill Mossdeep City and Steven will reward the player with items that can be transferred to the Pokémon Omega Ruby and Alpha Sapphire games. Clearing the first adventure also allows the player to transfer a Steelix or Glalie with its Mega Stone to the main games. After the first adventure, once a day, the player may talk to Steven to transfer the new Mega Pokémon or items to their full Omega Ruby or Alpha Sapphire game via Pokémon Link. There is no restriction on sending items to a full game, but if an Omega Ruby or Alpha Sapphire game has previously received a Steelix or Glalie from a Demo Version, it will not be able to receive another.
If the player faints during any adventure, they will be returned to Mossdeep City standing in front of Steven and the Pokémon Center, with everything reset to its default state. That is, no items will have been used or experience gained and all progress is reset; not even the mission type or the Trainer class or bonus NPC that was present are saved for random mission adventures. During the first adventure, however, losing the first battle at Mossdeep City or the battle with Steelix/Glalie does not cause the player to faint.
The player's default inventory consists of three Hyper Potions, three Full Heals, and (after the first adventure) the Mega Bracelet. Pokémon are only allowed to hold Mega Stones. The other items that Steven rewards the player with after completing certain adventures cannot be used within the Demo Version.
When the Demo Version is started with no saved data, it will randomly choose a theme: "red" or "blue". Throughout the Demo Version, this variable controls: the title screen's theme (Team Aqua for blue or Team Magma for red), the team that appears during missions or as NPCs at Mossdeep City, and the Pokémon that Steven rides (Latios for blue, Latias for red). This variable switches its value after every adventure that is completed, so that the adventures have alternating themes. It also switches its value if the player sends any of their rewards to their full Omega Ruby or Alpha Sapphire game.
The game's save data can be erased by pressing the Up, B, and X buttons simultaneously on the Demo's title screen.
The Demo Version features a unique scenario that begins in Mossdeep City, where Steven requests the player's assistance at the Mossdeep Space Center, where they are intercepted by a male and female pair of Team Magma or Team Aqua Grunts. The player is given a choice between a level 35 Grovyle, Combusken, and Marshtomp to help Steven fight them off.
After the battle, Professor Cozmo will inform the player and Steven that the evil team they just fought is seeking out a Pokémon with a recently discovered Mega Evolution. After Steven gets the coordinates of the unnamed island where the Pokémon is located, the player is allowed to explore Mossdeep City before returning to Steven to begin their pursuit.
After riding Latios or Latias to an unnamed island, Steven will tell the player to go forward, where they'll encounter wild Pokémon, several grunts from each team, and May, who came to investigate but was overpowered by the enemy. May will heal the player's team after the player's battle inside the cave. At this point, the player's Pokémon will have received enough experience to reach its final evolutionary form.
At the back end of the island cave, the player will find Aqua Admin Matt and Magma Admin Tabitha, who call a truce to take the player down. Before the battle can begin, however, Steven arrives, presenting the player with three gifts to shift the battle in their favor. First, he overwrites the player's Pokémon's status move with its respective Hyper Beam variant (Frenzy Plant, Blast Burn, or Hydro Cannon). Then he gives the player a Mega Bracelet and gives the player's Pokémon its respective Mega Stone to hold. He encourages the player to test these powers in the battle with the admins.
After defeating the admins and clearing Team Magma and Team Aqua off the island, Steven leads the player to a deeper part of the cave, where a single patch of tall grass houses the Pokémon sought by the two villainous teams. Steven gives the player five Poké Balls to catch the Pokémon. After a successful catch, the two Trainers return to Mossdeep City, the adventure ends, and progress is saved.
All adventures after the fifth adventure where the player does not have to battle an Expert can be one of three random types of missions. The second, third, and fourth adventures introduce the player to these missions and are not random, with the second adventure requiring the player to battle three Trainers, the third requiring the player to find a lost Pokémon or person (a Whismur, a Youngster, or a Twin—Gina, separated from her sister Mia), and the fourth requiring the player to find a Pokémon (a Poochyena, Skitty, or Shroomish). The lost characters will be partially hidden, while Pokémon inhabitants can be found by stepping to the left or right of an open patch in the middle of the tall grass. Immediately after an Expert battle adventure, any of these three missions may be randomly selected, but in the subsequent adventures, the mission type that was completed in the previous adventure will not be selected.
Trainers also appear in the random missions. The Trainers that appear will always be of the same Trainer class, with six different Trainer classes available to be randomly selected. The Trainer class that was present during the most recent random mission that was completed will not be selected, however. During Trainer battle missions, three Trainers will be present, but during the other missions, only two will be present.
A bonus NPC may also randomly appear at the destination Mirage spot during the random missions. These NPCs may be: Aarune, Lisia, an old man from Cave of Origin, one of the cardinal sisters from Mauville City, or a Scientist working for Captain Stern.
The location of the random missions is also randomly chosen from five different Mirage spots. Those locations are: the Mirage Islands west of Dewford Town (a beach) and north of Route 125 (an island with ruins), the Mirage Forest south of Route 109 (a circular path in a forest), and the Mirage Mountains northeast of Route 125 (on the cliffs of a mountain) and southeast of Route 129 (a small mountain). The expert battle always happens at the Mirage Cave west of Route 115.
All adventures after the first start with the player having the same party, consisting of a level 40 Steelix or Glalie and a level 36 Sceptile, Blaziken, and Swampert. If the player sends Steelix/Glalie to a full Omega Ruby or Alpha Sapphire game, however, it will not be part of the player's party. All Pokémon hold their respective Mega Stones.
During the adventures where random missions are possible, two special missions may also be selected.
One is a follow-up to the Demo Version's first adventure, where one of the evil teams has shown up again and Orlando must battle them with the help of May. It takes place at the Mirage Cave north of Route 124. This mission appears entirely randomly and is very rare, possibly on the order of Shiny chances (one in thousands). However, once the mission has been completed once, it may be started again at any time by selecting "Let's go somewhere secret!" from Steven's menu of options.
The other is a mission where the player investigates a strange man who ends up gifting the player a Nugget, which may be transferred to the Omega Ruby and Alpha Sapphire games. It takes place at the Mirage Island north of Route 111. When this mission's conditions are met, it preempts the other random missions so that it is the only one that will appear. Its only requirement is that fifteen days have passed since the first adventure was completed.