Ultra Beasts are from other planets and they want to conquer the Earth, excepting UB-01, which becomes Lillie and starts studying the Pokémon World to become stronger and face the UB, cause she is the weakest. She helps Kukui and she travels with you to become stronger . She has the key to stop Aether Foundation.

UB 02 beauty is Lusamine, and UB 02 expansion is Gladion. You fight against team skull like 90% percent of the game. Gladion wants to conquer the world before Lusamine so he joined team skull. Lusamine wants to create a perfect crieature to conquer the whole world, a failed experiment is Type: Null, but she manages to create this creature and names it Marshadow, unknown type with no weaknesses and it's attacks do 1,5 damage to every type.
Guzma catches Solgaleo/ Lunala and you get the other one. Guzma creates a League with Plumeria anD Gladion. After you beat it, Gladion becomes gooD and Team Skull vanishes.
You have to go to Aether foundation and battle against Lusamine and Marshadow. After this, she battles you in her UB form, that's when Lillie comes and gives you the last zygarde core and you battle her with Zygarde 100%, which is the third legendary, representing the middle between the sun and the moon, the Earth.
Then the UBS dissappear excepting Lillie. She goes on a journey to Kalos.
The other UBS are Wicke and Faba.