Pokémon Ultra Sun & Ultra Moon is the second main series Pokémon game in Generation VII and are enhanced and upgraded versions of Pokémon Sun & Moon. They take place in the Alola Region and feature a lot of changes from the original Sun & Moon including Pokémon that are not found in Sun & Moon.
Lots of new Pokémon forms are found in the game including ones based on Solgaleo, Lunala and Necrozma. This game also breaks all precedent by introducing brand new Pokémon
These games altered the story of the original Sun & Moon by involving the Ultra Recon Squad and their hunt for help to protect their world against the Pokémon Necrozma. With this, the story deviated more towards the end. It also altered the Island Trials, as well as various other elements of the game.
It also introduced a variety of new features including Mantine Surf and the Ultra Warp Ride, a way of going through the Ultra Wormholes to other worlds where Ultra Beasts and Legendary Pokémon live.