After arriving in Nacrene City and discovering that the Nacrene Gym is inside the city museum, our heroes meet a very scared man. His name is Hawes, and it turns out that he’s soon to become co-curator of the museum. He claims to have seen ghostlike apparitions recently, just as the museum is about to launch what is called the Secret Artifact Exhibition. The gang listens to the details of what has been happening, and Iris decides that an “angry spirit” is putting a curse on the museum. Cilan disagrees—he says that there must be a scientific explanation for everything.

While spending the night at the museum to investigate, our heroes and Hawes discover that a Yamask mask, which Hawes had mistakenly thought was a replica, was actually a real Pokémon! It was this Yamask that was appearing to haunt the museum, when really all it was trying to do was get its mask back! The Yamask was after Hawes for unwittingly taking the mask away, but the Pokémon forgives him and happily exits with its mask.

Then Hawes’ wife Lenora shows up. She’s the Nacrene City Gym Leader; Ash challenges Lenora to a Gym Battle, and she accepts! So now Ash’s second Unova region Gym Battle begins—hopefully it will lead to his second Unova Gym Badge!

Pokemon.com Synopsis