A Night in the Nacrene City Museum! (Japanese: シッポウシティ！博物館で大冒険！！Shippou City! The Big Adventure at the Museum!!) is the 14th episode of Pokémon the Series: Black & White, and the 671st episode of the Pokémon anime. It first aired in Japan on December 23, 2010 and in the United States on May 7, 2011.
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!
Ash and his friends finally arrive at Nacrene City, and Iris is amazed at all of the artwork on display. Cilan explains that Nacrene City is well known as the city of art and admiration because of the way the citizens essentially use their buildings as canvases. Though Ash is looking forward to his Gym battle, to which Cilan replies that the Gym is within the city's museum. However, when they arrive there is a sign outside informing them that the museum is closed. Ash tries to see if anyone is inside when Cilan notices a poster for a new exhibit that was scheduled to open that day. Ash knocks on the door once more and a man is heard screaming from inside the museum.
Hawes rushes out of the entrance doors, claiming that he was being chased by a Dome Fossil. The group finds the story to be a little bizarre, and inside they investigate only to find that the Fossil is back on its display. Hawes insists he was being chased, so Cilan suggests he tell them everything that happened and they go outside to a table to talk. Hawes explains that the night before, he and the staff finished setting up the secret artefacts exhibit late at night. He had stayed behind to make sure everything was in order before the grand opening when the lights went out, and when he made his way to the light switch he heard footsteps and sobbing. A flame then appeared and chased him out of the museum. The next morning he returned to the museum with the rest of the staff but they found nothing out of the ordinary, but he postponed the opening of the exhibit until the haunting was resolved to protect guests. When the rest of the staff left, he went back inside which is when he was chased by the Dome Fossil, and met Ash and his friends. Iris is convinced that the museum is cursed by a spirit, whereas Cilan is positive the events could be solved scientifically. Ash becomes frustrated, wanting to know whether or not he'll get the Gym battle he's been wanting, and Iris scolds him for being inconsiderate. Hawes explains that even if the museum was open, "Dear" was on a business trip. Ash questions who "Dear" is when the man introduces himself as Hawes, vice curator of the museum and husband to the Gym Leader, Lenora. The group then introduces themselves to Hawes and Cilan asks if they may have a tour of the museum.
Hawes shows them the Dome Fossil that was seen chasing him earlier, and explains that it was once a Kabuto. He also takes them to the world’s largest Dragonite skeleton, which excites Iris in particular. Next he shows them a meteorite that is said to hold cosmic energy. Lastly he takes them to the exhibit that was set to open that day which included suits of armor, and a collection of items retrieved from some ruins, including a well-made replica of a Cofagrigus which is said to reside in ruins. Iris states that she has a hunch of the cause of the curse, claiming it's her "sixth sense", however Cilan states she has no definitive proof. Ash then notices a mask inside a glass case and Hawes explains that it's a replica of a mask held by the Pokémon Yamask, though Iris grows increasingly suspicious. Hawes guides the group through to the library, which allows guests to research the museum artefacts. Hawes asks if they figured anything out during the tour. Cilan explains that nothing seemed out of the ordinary, and Iris states that she couldn't come up with a reason for the bad vibe she got. Ash just complains of being hungry, and Cilan asks if they could spend the night at the museum to experience any phenomenon for themselves, which Hawes accepts. He is inspired by their courage despite being children, and Iris remarks that Ash is the only child among them.
That night, the group set up camp within one of the exhibits and Hawes apologizes for the crudity of the manner they're forced to sleep in, and Ash replies that they're use to it. Outside, Team Rocket is seen waiting in a dark alley when Pierce appears with their next orders. He asks if they have investigated the museum, and after Jessie confirms he explains that their mission is to swap the meteorite within the museum with the dummy they were given at the train station. Pierce explains that their ultimate target is and isn’t for the meteorite, though the cryptic statements leaves the trio perplexed.
Back in the museum, Iris is attempts to wake up Ash after feeling a strange chill, though Cilan has his doubts. Suddenly a dark mist appears. Hawes wakes up and leaps to his feet in fear and Iris claims the spirit is trying something, though Cilan claims it must be the ventilation system's broken. As a suit of armor appears, Cilan is convinced it's merely low frequency waves, until the suit draws its sword and he claims it's being controlled by strings like a puppet. Regardless, Ash has Pikachu use Quick Attack which causes the suit of armor to fall apart. It then reassembles itself, so Pikachu strikes it with Volt Tackle which dismembers the suit of armor yet again and releases a flame from inside. It then chases Hawes around the room and Ash calls out his Oshawott and orders it to use Water Gun. The head of the suit then reanimates itself and chases after Hawes, so Ash has Pikachu take care of it once and for all with Iron Tail. Iris claims she thinks the spirit is after Hawes, and he states that he hasn't done anything to make a spirit angry when it begins raining inside. Cilan, however, dismisses the phenomena as a problem with the sprinklers.
It stops raining after only a few seconds and sobbing is suddenly heard coming from the next room. As Iris and Cilan argue over the source, Ash tells them that there's no way they'll know for sure until they see for themselves. In the next room Ash notices that the mask from earlier is actually crying. As Cilan tries to explain how it's producing tears, the mask levitates and starts bashing itself against the glass. When the Dragonite skeleton also becomes animated, Cilan concludes that he cannot explain this one. As Dragonite begins punching the glass of the display, Cilan sends out his Pansage and has it use Bullet Seed which causes the skeleton to fall apart, as well as produce a strange cry. Hawes opens the display to check the condition of the mask when it suddenly flies across the room and places itself on Cilan's face and takes control of him. Iris remarks that Cilan is now possessed, as he speaks in an auspicious tone and fires two balls of fire at the Iris tries talking to the spirit, and it replies that it is angry because the mask was caged, and that it is precious.
Lenora suddenly appears in the doorway and explains that the mask must be the real mask of a Yamask. Hawes is overjoyed to see his wife has shown up just in time. Lenora asks Hawes where he first got the mask, and he replies that he found it lying on the ground as the staff workers were carrying in the items for the exhibit. He assumed it was thrown in as an additional artefact, despite having only ordered the Cofagrigus replica. The mask then reassembles the Dragonite skeleton and Lenora calls out her Watchog which illuminates the room with a blinding light. Ash scans the Watchog with his Pokédex as Lenora requests that Yamask shows itself. The mask then removes itself from Cilan's head and floats towards the ceiling where a Yamask appears. Ash scans the Yamask, and Lenora explains that Yamask must have been among the items for the exhibit and must have somehow dropped its mask, which Hawes then took as a replica and put on display. She adds that when Yamask realized the whereabouts of its mask, it must have thought it was taken from it. Yamask confirms Lenora's theory as true, and Cilan realizes that all the strange events were caused by Yamask using Psychic. Hawes runs to Lenora's side and sincerely apologizes to Yamask for the misunderstanding, which the Yamask accepts with forgiveness. With the Yamask happy again, Ash exclaims that Yamask is actually a cute Pokémon and Cilan states it has a mysterious taste.
At sunrise they bid the Yamask farewell. Ash claims that it's the perfect morning for the day of his second Gym battle. He formally introduces himself to Lenora, and Lenora gladly accepts his challenge.
In one scene, Ash and his friends are standing on a sidewalk, but in a later scene, it switches to a blue road.
When Hawes wakes up and is startled by Haze, the buckle on the back of Cilan's vest is completely missing.
When Lenora is calling out Watchog, the beige and orange colors on her headband swapped the places with following orders, orange goes with beige and ends with an orange color. This error was corrected only in the international version, while this error is still presented in the Japanese DVD releases.