It first appeared as a Taillow, landing on Cameran Palace and then transforming into Treecko. It continued transforming, including into an Aipom to play with Ash's Pikachu. Ash and his friends released their Pokémon to play with what they thought was an Aipom.
After taking them to an attic, Kidd Summers sent her two Weavile after it. To escape, it teleported, along with Pikachu and Meowth. It then transformed once again into a Pidgeot and flew the Pokémon to the Tree of Beginning. While Ash and the others were looking for Pikachu, Mew kept it entertained with toys.
When the Tree of Beginning's automatic defense system absorbed Ash, Mew saw how distraught Pikachu and Ash's other Pokémon were over the loss of their Trainer. As a result, Mew restored all of the humans who had been swallowed, but this act nearly killed Mew as it sent the Tree of Beginning's biological parts into shock and causing it to start to crumble. Lucario stepped in and was able to save Mew, but ended up sacrificing itself.
Mew is believed to still reside at the Tree of Beginning. It is last seen playing with a Bonsly.
Mew is said to have a very playful nature. It comes now and again to the palace to get some of the toys. It brings these to the Tree of Beginning, where it would play with them. At the end of the film, Sir Aaron's gloves could be seen among its collection.
