Location: Aether Foundation

The first Ultra Beast Encounter is with the mysterious UB-01 when you first are in the Aether Foundation. Due to the jamming signal the Aether Foundation has set up, this Pokémon cannot be captured and is at Level 27. Its Defense is boosted.

After this encounter, you then see UB-01 again when Lusamine forces Cosmog to create an Ultra Wormhole. This causes Ultra Wormholes to open up across the Alola Region, notably seen with Buzzwole/Pheromosa appearing and facing off against Tapu Koko. After that, you'll see a group of Nihilego in Ultra Space, where one temporarily merges with Lusamine.

After defeating the league and becoming the first Alola Region champion, you will receive an Enigmatic Card from a mysterious man outside your house requesting to go to the motel in Route 8. Follow the instructions and you will meet Looker and his boss, Anabel. They will request your help to hunt down the Ultra Beasts, as many have managed to get through to the Alola Region and are causing havoc.

First, Anabel will challenge you to a battle. After defeating her, she will suggest going to the Aether Foundation to speak with Wicke. Wicke will give you the prototypes of special Beast Balls. These special PokéBalls have a high capture chance when used on the Ultra Beasts, but have a very low capture rate when not used on Ultra Beasts.

Location: Route 8 - Diglett's Tunnel/Wela Volcano Park

Return to Route 8 and Looker will burst into the motel saying that UB-01, the Pokémon Nihilego, has been spotted in Wela Volcano and Diglett's Tunnel. They request for you to go hunt it down and capture it. When you get there, it can be found in the grass/cave just randomly. You'll know it's in the area as the music shall change.

When the battle begins, Nihilego will be absorbed by an aura which boosts its Special Defense by two stages as standard. It is Level 55 and Rock/Poison-type.

Location: Route 2 - Melemele Meadow (Sun), Verdant Cavern (Moon)

After capturing Nihilego, when reporting to Anabel, Looker will come in and say there has been a sighting in Melemele Island so they suggest regrouping in Route 2. When there, the Ula'ula Island Kahuna, Nanu, will appear and inform you of a sighting he has been made aware of. He will inform you that UB-02, the Pokémon known as Buzzwole (Sun) & Pheremosa (Moon) have been seen in the nearby area, and that there are two Buzzwole or four Pheromosa. It's your task to capture them all. Buzzwole will be in Melemele Meadow while Pheremosa is in Verdant Cavern

When the battle begins, Buzzwole will be absorbed by an aura which boosts its Defense by two stages as standard. They are Level 65/60 respectively and Bug/Fighting-type.

Location: Route 8 - Lush Jungle and Memorial Hill

After capturing Buzzwole/Pheromosa, when reporting to Anabel, Looker will come in and say there has been another sighting in Akala Island so they suggest regrouping in Route 8. When there, you meet up with the final Trial Captain, Mina, who informs Looker that she has seen UB-03, otherwise known as Xurkitree. Before she tells you where it is, she challenges you to a battle. Defeat her and she will inform you that Xurkitree has been seen in Memorial Hill and Lush Jungle , and that there are two of them. It's your task to capture them all.

When the battle begins, Xurkitree will be absorbed by an aura which boosts its Special Attack by two stages as standard. It is Level 65 and Electric-type.

Location: Route 13 - Route 17 (Sun), Haina Desert (Moon) & Malie Garden

After capturing Xurkitree, when reporting to Anabel, Nanu comes back and states that more Ultra Beasts have been spotted on Ula'ula Island so they suggest regrouping in Route 13. When there, the Ula'ula Island Kahuna, Nanu, will appear and inform you that UB-04, the Pokémon known as Kartana (Sun) & Celesteela (Moon) have been seen in the nearby area, and that there are four Kartana or two Celesteela. It's your task to capture them all. Kartana will be in Route 17 and Malie Garden while Celesteela is in Haina Desert and Malie Garden

When the battle begins, Kartana will be absorbed by an aura which boosts its Attack by two stages as standard. They are Level 60 and Level 65 and Grass/Steel and Steel/Flying-type respectively.

Location: Seafolk Village - Resolution Cave

After capturing Kartana/Celesteela, then when you return to report on the situation, Looker explains how 10 years ago he and Nanu were working on an operation where they were hunting down Ultra Beasts, using a 'Faller' as bait for the Ultra Beasts, since they have Ultra Wormhole energy, and that Anabel is a Faller, having arrived in Alola having lost her memory after seemingly being pulled through an Ultra Wormhole. They state that you are also a Faller due to your travel to Ultra Space. They then say the final Ultra Beast has been seen on Poni Island, and that they should go to the Seafolk Village. When there, Nanu challenges you to a battle once more to make sure you are ready. He then states it can be found in Resolution Cave. It can be found in the basement

When the battle begins, Guzzlord will be absorbed by an aura which boosts its Attack by two stages as standard. It is Level 70 and Dark/Dragon-type respectively.

Location: Seafolk Village - Aether Paradise

Return to the restaurant and Looker & Anabel will congratulate you on a job well done and suggest reporting to Wicke about having contained all the Ultra Beasts.

When you go there, Looker bursts in and informs you of a mysterious black creatured zzipping across the sky of Alola. He insists it's an Ultra Beast but Anabel says there's no word of more Ultra Beasts. They say about how it must be Looker being tired. Anabel leaves to meet Nanu and Looker goes to the motel on Melemele Island.

The mysterious Pokémon turns out to be the Legendary Pokémon Necrozma