WARNING: The following contains spoilers for Season 1, Part 2 Episode 4 of Pokémon Journeys: The Series "A Chilling Curse!" now streaming on Netflix.
Ash has caught, released and befriended a variety of Pokémon along his journey. There are only a few types he has yet to catch, those being Psychic, Fairy and Ghost. Ash sometimes fights alongside the Psychic/Fairy-type Mimey, but Mimey is his mother's Pokémon, not his. Ash also teamed up with a Haunter during his battle against Saffron City Gym Leader Sabrina, but he never actually caught the Pokémon. But finally, Ash has a formidable Ghost-type of his own, and it's even from the same region as him.
Gengar is back, terrorizing the lab assistants with spooks and pranks. Mimey complains to Ash that the Gengar is knocking over tea, breaking brooms and generally causing a mess. As Mimey informs Ash of the mayhem, Gengar gets further frustrated, throwing dolls, Shadow Balls and even Pikachu everywhere. Gengar appears to be quite attention-hungry, but the reason for why is unclear. The Professor explains that he bought the mansion because he thought all the poltergeist activities like flashing lights and door slamming would be fun, but with more people at the lab now, it seems Gengar is rowdier and angrier than ever.
Of course, being a Ghost-type, Gengar has some chilling Pokédex entries. According to its Black & White entry in particular, Gengar "the leer that floats in darkness belongs to a Gengar delighting in casting curses on people.." Perhaps Goh has also read these entries, as he teases Ash with the rumors of Gengar's curse and that it may have cursed its previous trainer . All of a sudden, Ash gets some serious goosebumps and his chair leg even breaks. In a dazed confusion, Ash drinks water out of a vase too. Throughout the day, his luck goes from bad to worse. He falls down the stairs, dumps way too much seasoning on his pasta, gets run over by Pokémon at the park and steps into a random pitfall in the ground. This entire time he tries to shrug off the idea of being cursed, but stepping into a literal trap sets him off.
Freaked out by all these mysterious occurrences, Ash goes out for a run as Gengar follows in his shadow. While turning a corner, Ash crashes into another trainer, one Gengar instantly recognizes. After getting knocked down by Ash, he jokes about being cursed by a Pokémon three years ago and leaving it behind for good. This Pokémon the cruel trainer left behind was none other than Gengar.
Gengar's loyalty to its trainers is reminiscent of an earlier episode of Indigo League, when a Charmander refused to leave the spot its trainer abandoned it in. Likewise, Gengar waited in the same spot for years and during that time developed massive distrust towards other humans. After remaining in the same spot for so long, it entered an empty mansion and continued to wait in there. One day, some people open the door giving Gengar momentary hope that its trainer had finally come back, only to realize they were just people interested in buying the property. Angry at humans and the world, Gengar remains in the mansion to haunt it, knocking down random items whenever people come by to view the home.
Annoyed at the trainers' abuse, Ash demands he take Gengar back. He ignores Ash, saying he should have never caught that Pokémon. Gengar loses it. It throws a Shadow Ball at its trainer but Pikachu blocks the attack with Electroweb. Frightened, the trainer runs off, as does Gengar (in the opposite direction).
Meanwhile, Team Rocket overheard Gengar's dramatic story and try to schmooze it over to their side. Gengar shrugs their advances off, but to no avail, as Team Rocket takes the Pokémon by force, Ghostbuster style. Ash and Pikachu catch up to Gengar, free it, and with an outstretched hand, Ash asks Gengar to join him in battle. They form a double team against two Dusclops, mighty foes but no match for Gengar's Night Shade and Shadow Ball.
After blasting Team Rocket off, Ash faints in Gengar's arms. He was never cursed, just sick with a fever. Ash gets some rest and wakes up to a Pokéball and an apple at his bedside. Shyly standing by, Gengar asks to be Ash's friend. Ash accepts and thus catches his first-ever Ghost-type Pokémon. A mischievous fellow quite like the Haunter he teamed up with so many eons ago, but also strong and independent, Gengar is sure to be a powerful addition to Ash's team.