It's the third day of Professor Rowan's Pokémon Summer Academy, and he's sent the campers to observe Water-type Pokémon living in a nearby lake. There are just two main rules: no catching Pokémon, and no leaving the area! As the Trainers begin searching for Water-type Pokémon, Pikachu sees a flash of light and spaces out for a moment, but quickly recovers. Ash and his friends continue to observe Pokémon until Angie, another Trainer on Ash's team, notices that her Shinx is missing. And Piplup falls into a brief trance after it sees the same flashing light as Pikachu!
Conway thinks this is related to the legend of the lake, which says that Pokémon vanish when the blue light flashes. Ash and his friends search for the source of the light, and they eventually stumble into a cave with an eerie green glow. At the back of the cave, Angie's Shinx is standing by a glowing pool of water, and the glow is coming from the fins of a Lumineon. As our heroes wonder what an ocean-dwelling Pokémon is doing in this cave, Jessie—now pretending to be another Summer Academy camper—barges in and declares she's going to catch Lumineon.
Of course, that's against Professor Rowan's rules, so Conway uses Slowking to drive away Jessie and her Yanmega. All the commotion frightens off Lumineon, but Ash and Buizel dive into the water and spot an underwater tunnel. That must be where Lumineon went—the tunnel lets it swim from the ocean to the cave, where there's lots of seaweed to eat. The flashing lights on its fins hypnotize the "vanishing" Pokémon, too! Professor Rowan is pleased to hear our heroes' discovery, but he also deducts points from everyone who broke his rules by entering the cave. It looks like Ash and his team will have to make up some points if they want to win at the Summer Academy!
On the third day of Pokémon Summer Academy, the students are tasked with studying a Water-type Pokémon from the lake and presenting their findings. Professor Rowan and lab assistant Yuzo remind the students that the lake Pokémon are not to be caught and that no one is to leave the area.
While standing by the lakeside, Ash and Pikachu see a flash of light coming from across the lake, and Pikachu suddenly becomes hypnotized and begins to walk towards the light, but Ash stops him immediately. Intrigued, Ash and Angie find two Lanturn-shaped pedal boats and use one to explore the far side of the lake, while Dawn and Brock follow using the other boat. While crossing, Ash and Angie stumble across a Dewgong and a Sealeo. Ash decides on the Dewgong while Angie chooses to draw Sealeo, but they begin to quarrel when both Sealeo and Dewgong move the opposite direction, causing Ash and Angie to bump into each other while trying to keep a close eye on their Pokémon. Meanwhile, Dawn chooses a Gorebyss and Brock chooses to sketch a Chinchou. After Ash and Angie consider their finished drawings to be terrible, Ash decides to get up close to the Pokémon to make the research better, and they all dive into the water to be able to interact with the Pokémon. They return to the bank to begin writing a report on each of their Pokémon.
Shinx hypnotized by the light
As the group is talking, a blue flash comes from across the lake and Angie's Shinx becomes hypnotized and walks off. After a few minutes, the group realizes that Shinx is missing and they start looking for him. The light flashes again and Piplup becomes hypnotized, but Dawn stops him from wandering off. Ash comments that the same thing happened to Pikachu earlier, and Conway appears and tells a legend about a blue light that causes Pokémon to vanish when it flashes. The group decides to investigate the light, and despite the possibility of losing points, Conway follows to spend more time with Dawn. They find a cave and rush inside after it begins raining heavily. Conway looks deep into the cave and spots the odd light again, so the group venture farther into the cave. They enter a green-lit cavern filled with a lake, and the mysterious light swims closer to them. The source of the light leaps from the water, revealing itself to be a Lumineon. Conway adds that Lumineon uses the light of its tail fins to hypnotize Pokémon and draw them in. Angie spots Shinx inching closer to the water, but gives him a shake to wake him up from his hypnosis. Brock admits that Lumineon aren’t normally in caves, so he suggests they try to figure out this mystery and make it their report topic.
Thanks to James and Meowth, Jessie who is disguised as fellow camper Jessilinda, finds out about the Lumineon. She is determined to catch it, despite the others reminding her that they were instructed not to catch any of the Pokémon. Jessilinda sends out her Yanmega, though Conway's Slowking uses Psychic to toss Jessie and Yanmega out of the cave. The group realizes that Lumineon is gone and must have been scared off by the battle. Ash and Buizel head underwater to search for Lumineon while Brock discovers that the Lumineon had been eating some algae on the rocks in the cave. Ash discovers an underwater tunnel, and Brock deduces that the tunnel must connect the lake to the cave, and since Water Pokémon like algae, Lumineon was traveling to the cave to eat the algae.
When the group returns to the class they present a report on Lumineon, and although they get a perfect score of 10, Professor Rowan deducts four points because they left the lake without permission, breaking a rule that had been stated earlier that day. Jessie and Conway also lose four points, but Conway simply states that it was fewer than he expected. Although Ash and friends are disappointed for losing the points, they admit it was worth seeing Lumineon. They conclude day three of the Summer School by cleaning the classrooms.
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