The Lost World of Gothitelle! (Japanese: スカイアローブリッジとゴチルゼル！ Sky Arrow Bridge and Gothitelle!) is the 21st episode of Pokémon: Black & White.
Ash and co. are well on their way to Castelia City via the Skyarrow Bridge, but a Gothitelle blocks the way. Who is Sally, and what connection does she have with Gothitelle?
Cilan reads that if they cross over the Skyarrow Bridge, the longest bridge in Unova, they will find themselves in Castelia City. Coming near it, the heroes see it is much larger than they anticipated. Ash runs to call Prof. Juniper and nearly bumped into a woman. He lets her pass, though Iris and Cilan tell he should calm down. Ash calls Prof. Juniper and reports he caught Sewaddle. Juniper is excited and sends him Pidove back for Sewaddle. Suddenly, the heroes see the woman Ash encountered, who views the pictures. She remembers there was water taxi to transport people around. Iris asks if she lives here, but the woman denies and walks away.
The heroes come out, only to find a fog. Jenny arrives and tells the area is known to be foggy. She sends Swanna, who tries to defog the area, but fails. A car passes by, so Jenny goes to stop it, as they shouldn't drive in a foggy area. The heroes walk pass the bridge, but notice the fog getting thicker. Suddenly, they get attacked by Gothitelle's Psybeam. Gothitelle does not want them to pass the bridge, so Ash sends Snivy, who uses Leaf Blade, but Gothitelle protects itself. Gothitelle uses Psybeam, so Snivy uses Leaf Storm, countering the attack, causing the heroes to be caught in the blast and they fall down. Ash calls Snivy back and notice the bridge being under construction. A young girl asks them if they came for the water taxi. The heroes decide to enter it, hoping to get some answers.
The heroes notice Gothitelle, who seems happy. The heroes board the water taxi, while Gothitelle uses Psychic to unbind the rope. As they travel, the girl offers them food, which the passengers take and eat. The passengers heard the water taxi is being closed down, though the captain admits he can't do anything about it. Cilan thinks they traveled into the past when the Skyarrow Bridge was being constructed. The heroes go out of the ship, but see they are at the point where they started. Cilan believes they are in a loop; even if they go on the water taxi, they will still be at the same point where they started. While the heroes wonder what they should do, Gothitelle uses Psychic, helping the girl carry the juice for the passengers.
The heroes decide to investigate about Gothitelle. They introduce themselves to the girl, whose name is Sally. The heroes decide to have another ride at the water taxi, as it will be closing down. They wonder if Gothitelle is her Pokémon. Sally tells it is a wild one, though it did help them carry the stuff on the taxi one day and continued to do the work. Gothitelle and Sally run off, while Iris notices Gothitelle being happy. Cilan suspects Gothitelle created this world for itself, for it even created the fog, which couldn't be removed by Swanna's Defog. Cilan tells a Pokémon creates a lot of memories, like Gothitelle did. Sally and her father, the captain, say farewell to Gothitelle, who moves away, so the heroes follow it. At the bridge, the heroes tell Gothitelle that they want to get out of this memory world, though Gothitelle attacks them. Cilan advises to have a battle, for they might learn something to get out.
Ash sends Snivy, who dodges Psybeam and retaliates with Vine Whip. Gothitelle uses Protect and dodges Snivy's Leaf Blade using Double Team. Gothitelle uses Psybeam, hitting Snivy, who uses Leaf Storm. Gothitelle protects itself and uses Psybeam against Snivy's Leaf Blade. Gothitelle uses Psychic, levitating objects to attack the heroes. However, a woman appears that Ash nearly bumped into, introducing herself as Sally. Everyone is shocked, though Sally knows this is the memory world Gothitelle created. When the Skyarrow Bridge was opened, Gothitelle was very sad, for it is all over. The following day, Sally offered Gothitelle to come with them, who accepted. After moving into a bigger town, Sally went to a medical school, but several years later Gothitelle disappeared. Sally knew this is where Gothitelle went to, but reports she is now a doctor, going to many hospitals to work.
She is touched, as Gothitelle values these memories as much as Sally does. Gothitelle is pleased and removes the fog, so the heroes and Sally are back at Skyarrow Bridge, which is built. Sally sees Gothitelle at the top and promises to visit it once more, who vanishes in a fog. Cilan believes after seeing Sally grown-up, Gothitelle put its memories to rest. Jenny comes and sees the fog is cleared, so goes to open the bridge. At the bridge, the heroes admit it was nice to see the water taxi active before the bridge was finished. The heroes bid farewell to Sophie and see Pokémon value their memories. Ash speeds up towards Castelia City, with Cilan and Iris following him.
The "Who's That Pokémon?" is Gothitelle.