As with previous installments, the games follow the journey of a young Pokémon Trainer, taking place in the Alola region — based on Hawaii.

As of 31 March 2023[update], a combined total of 9.15 million copies have been sold worldwide, ranking them as the ninth-best-selling Nintendo 3DS titles of all time.

While set in an alternate version of the Alola region, the mechanics and graphics remain largely the same as Pokémon Sun and Moon, with the primary differences being its modified storyline now including the Ultra Recon Squad.

[2] Multiple main characters featured in Sun and Moon, such as Lusamine and her children, return in the game with significant changes.

[10] Similar to Sun and Moon, the player character is Elio or Selene, who is moving to Melemele Island in Alola with their mother.

During their travels in Alola following the region's traditional island challenges, they complete trials which involve battles with powerful Pokémon known as Totem Pokémon, and encounter numerous groups — a villainous one known as Team Skull, led by a man named Guzma; a more charitable one known as the Aether Foundation, led by a blonded woman named Lusamine; and another called the Ultra Recon Squad, who came from a different dimension, the Ultra Megalopolis, where Necrozma has stolen its light.

Necrozma then absorbs the legendary Pokémon, gaining its Dusk Mane or Dawn Wings form in the respective version, and unleashes the Ultra Beasts upon Alola before fighting the player.

After completing these trials, the player proceeds to battle a newly established Elite Four and later defeats Hau to become the first-ever Alola League Champion.

Battling through villainous team leaders from the past games—Maxie and Archie from Ruby, Sapphire, and Emerald; Cyrus from Diamond, Pearl, and Platinum, Lysandre from X and Y, and Ghetsis from Black, White, Black 2, and White 2—the player finally encounters Giovanni, who leads Team Rainbow Rocket and has a Mewtwo at his disposal.

[12][6] In a separate interview, Ohmori also described an idea to develop the Ultra installments formed late during the development of Sun and Moon, with the titles intended to take advantage of the momentum gained by the Pokémon series following the massively successful release of the mobile game Pokémon Go.

[13] In the post-game, the games include a tribute to former Nintendo CEO Satoru Iwata, mentioning his role in the development of Pokémon Gold and Silver.

[25] On the other hand, Allegra Frank of Polygon criticized that the aforementioned changes were only present at the end, with the bulk of the gameplay being the same as its predecessor.

[29] Following release, the two games sold 1.2 million physical copies—excluding digital copies purchased from the Nintendo eShop—within the first three days on sale in Japan.

[33] As of 31 March 2023[update], a combined total of 9.15 million copies have been sold worldwide, ranking them as the ninth-best-selling Nintendo 3DS titles of all time.