Our Friend Henger Forever Monster Farm: Enbanseki no himitsu : Season 1 Episode 23

The Searchers discover an ancient city, perfectly preserved by Hengers. But, the robot caretakers are programmed to prevent anyone from entering it! When the Hengers take Holly's pendant, Genki and his friends must trust a malfunctioning Henger, who was banished from the city because of his free will, to find a way inside.