Meltan were originally sealed within the crated located in underwater near Poni Island, until for some reason, the seal was broken and currently only three Meltan who are briefly seen walking underwater.
When Ash and his friends were finished with their studies at Poni Island for their preparation at the upcoming Alolan Pokémon tournament and return to Melemele Island, there are more than three Meltan who are sneaking on board, hiding in a shadow within the ship’s packaging room, eating few metalic substance and making ring sounds.
When they arrived on Melemele Island, all the Meltan were in a warehouse before they went into the sewer, after they ate the manhole cover. However, they ended up getting separated into two different groups when they were startled by a wild Rattata. One group ended up going into Team Rockets base while the other group ended up in the Ultra Guardians base inside the Pokémon School. Both groups ended up causing trouble for Team Rocket and for Lusamine's Clefable as they been eating all the metal they can find. The group of Meltan that was at the Pokémon School went upward and into the classroom. Seeing Sophocles toolbox, the group of Meltan ate the metal until they were discovered by Ash and his classmates when Lillie, who accidentally mistaken it for a bolt, picked up one of them. The group of Meltan were observed by Ash and his classmates along with Professor Kukui until they ran away, after they panicked from Kiawe's Marowak flame on his bone when he was doing a greeting dance for them. While one of the Meltan has taking a liking to Ash's Rowlet and followed him into Ash's backpack, the group of Meltan were trying to find a way to signal the other group of Meltan of their location until they went up to the school bell while the other group went up on top of a tree where they saw each other. Both groups reunite at the Ultra Guardians base, much to Clefable annoyances.
The group of Meltan, minus one, were causing more trouble and chaos for Clefable due to eating any metal that they could find. However, the group of Meltan were put in a box by Clefable and taught them how to behave better by her as well. The group of Meltan were brought up to the classroom by Clefable to show Ash and his classmates with Professor Kukui of what they can do when they listen to her then gave some metal as an reward. The group of Meltan reunites with the one that has been watched over by Ash's Rowlet. However, the group of Meltan had to say goodbye to their friend when it decided to stay with its new friend but they understood and approved of its decision.
Under direction of Clefable, they began to work for Aether Foundation to supply gadgets for the Ultra Guardians. When greeted and asked by their old fellow Meltan friend, who is now part of the Guardians, they reply to have love their jobs.
When Ash and friends were on a mission, the Meltan somehow multiplied. When Ash's Pokémon arrived at the Ultra Guardians base Meltan then show them how they multiply themselves which impressed Ash's Pokémon before they went on a mission of finding their trainer, as well as their teammate Torracat. 
At the Ultra Guardians base, Meltan were called by their friend to come join it at the Alola League. After hearing their friend call, They to went to the Manalo Conference. The Meltan hordes then arrive at League after Semi finals were over, when they were traveling underwater. Finally arriving at Manalo Conference tonight before the final begins tomorrow, The Meltan hordes begin an evolution ritual with their friend, becoming a single Pokémon known as Melmetal.