Mass Hip-Po-Sis! (Japanese: 迷子のヒポポタスを助けろ！ Save the Stray Child Hippopotas!) is the 44th episode of Pokémon: Diamond & Pearl.
The heroes' Pokémon are all fed up and ready to go. Pikachu mimics to Ash Turtwig is gone. They go to search and find Turtwig, though Turtwig pulls Ash's leg, as something is wrong. They see a Pokémon stuck on a cliff and go to help. Ash goes down and almost fell, though Brock pulled him. Ash offers the Pokémon to go with him, but it bites him. Pikachu comes down and tells the Pokémon Ash is there to help it, so the Pokémon comes to Ash and they are pulled up. Brock recognizes it is a Hippopotas.
Brock gives it some food for bravery, but when the Hippopotas eats it, it does not like the food. Hippopotas wants some fruit and Ash takes a fruit from a nearby tree and the Hippopotas gladly eats it. The heroes are glad to have helped it and leave, but Pikachu spots it wanders to them. They see it wants to follow Ash. The Sinnoh Now staff appear out of nowhere to report the story. They are to report the story of the missing Hippopotas, as they like to be in dry places with the herd. Rhonda sees the local map and points at a place where the herd is. The heroes go to get the Hippopotas back to the herd.
As they run, they all fall in the hole and Hippopotas is being taken by Team Rocket. The Hippopotas burps and a bubble pops near Meowth. Ash sends Turtwig, who uses Razor Leaf to cut the rope and allowing Ash to get it. The Hippopotas burps again and the bubble pops near the others. The heroes run away, while Team Rocket searches for them. Meowth went to sleep, as Hippopotas used Yawn. James thinks the Hippopotas would be good for the boss during his golf matches, where the Hippopotas would push the ball from sand or even make sand beaches for him. Pikachu is on Dawn's shoulder, as Hippopotas is on Ash's head. Brock falls to sleep as well, as he was affected by Yawn. They see Team Rocket looking and hide in the bushes. Though they forgot Brock, Croagunk pulls him to them. Rhonda has also problems, as her microphone holder is sleeping as well.
Brock woke up and the heroes continue to go. The heroes see the road is closed due to construction repairs. Hippopotas eats a fruit, but it was a trap made by Team Rocket, who pulls it up. Still, Hippopotas uses Sand Attack and follows with Sand Tomb, causing the balloon to fall down. Ash sends Aipom, who uses Focus Punch to blast them off. Hippopotas uses Yawn and affects Dawn, so Brock has to carry her. They came to a bridge and have to cross it, despite Hippopotas hates water. Team Rocket appear once more and use a net to get Hippopotas. Ash grabs the net and it is broken, causing the Hippopotas and Ash to fall into the river. Brock sends Croagunk, who uses Poison Sting to blast Team Rocket off once more. Dawn sends Buizel, who gets Hippopotas and carries it to shore.
Hippopotas got sick from the shock, so the heroes carry it to the Pokémon Center. Joy checks it and with a lot of sand, medicine and rest it will be fine. Joy worries if there will be enough sand, so Ash volunteers to get some. With Staravia's help, the heroes found and got a bag full of sand. Ash trips over and they collect the sand back, then return to the Center where Hippopotas is covered in sand. Hippopotas is better and eats the food. Brock flirts with Joy, but gets jabbed. The heroes continue to get Hippopotas back and see two people who offer dry sand (Team Rocket). Hippopotas yawns, affecting Jessie. Ash places Hippopotas on the ground to try the sand, though Jessie gets it and immediately go to the balloon.
Hippopotas managed to get out of the bag, angering the heroes for another attempt at stealing. James sends Cacnea and Ash his Turtwig. Turtwig dodges the Pin Missile and bites Cacnea. With a Razor Leaf and Tackle, Team Rocket is pushed to the balloon and (again) blasted off by Thunderbolt. The heroes came to the site where the Hippopotas wandered - some footsteps can be seen in the sand. As they came to the end of the footprints, they see nothing. Hippopotas shouts and his fellows emerge from the sand, so it goes to them. Ash throws his last fruit and the Hippopotas eats it. The Hippopotas yawns and affects Ash and Pikachu. While they are carried, Dawn thinks Hippopotas was grateful for Ash's help he deserves to rest.
- Brock opened his eyes in this episode.
- This episode's name is a reference to "mass hypnosis".
A part of Cacnea's head was colored wrong when Turtwig bit it. A scene later, when Turtwig tackled Team Rocket, the same error occurred.