Zombie Gas! is the 21st episode of Season 7.
A strange green and stinky gas in Hub City has everyone turning into zombies, which the Hub Squad must put a stop to before Hub Island is infested with zombies!
The episode starts at Hub High in the Math Class where Mrs. Creecher was teaching the students about equations. Lief, unlike his other classmates who were bored and then sleeping in, was the only one taking notes and paying attention to the lesson, that is until he sees his friend, Dennis, playing an app game called "Pants VS Zombies" on this iPad.
Putting his pencil down, Lief decides to take a break from taking notes and watch Dennis play his game. After Dennis won the level, he and Lief both shouted together which had caught everyone's attention, including the teacher's. When the boys realized that they've been shouting, Dennis decides to make an excuse for a bathroom break so he can beat the boss battle and with a heavy sigh, Mrs. Creecher lets him and Lief go.
Later in the boys bathroom, Dennis and Lief find Guy, Curly, Pieface, and Yawp and Dander (who were suppose to be in the school gardens with the other school pets) were also playing the same game. Suddenly, the lights went out which had the boys freaked out. So to calm his friends down, Pieface suggests the emergency lighting will go on. But instead of an emergency lighting, it was a spooky lighting and then there was this creepy zombie-like moan that scared Yawp so much that he jumped into Dander's arms. But what they didn't notice is the reflection on the mirror that looked like a zombie.
Meanwhile in the corridor, a spooky green fog creeps around the school. The fog that looked like the one that turns people into zombies like in the app game. Soon, Mrs. Creecher opens the door, wondering what was taking Dennis and Lief so long but while she was waiting for them, she noticed the green fog that was coming to her and when she inhaled it, she turned into...a zombie! At least it was happening in the school. But it wasn't just happening at the school, it was happening all over Hub City and it was turning everyone (person, animal, pony) into zombies too!
Meanwhile, back in the boys bathroom, Dennis, Pieface, Curly, Guy, Yawp, and Dander were still playing their games while Lief was watching. Four hours later, Strawberry Shortcake and Komodo appear to tell them that it was time to go back to the classroom right away before Mrs. Creecher gets furious. Luckily, the boys finished their game and headed back to class just when something caught Yawp's attention! He sniffs the air and then gags that he holds his nose but Lief tells Yawp that the school janitor is probably just fixing some toilet pipes. When they came back to the classroom, they find Mrs. Creecher who was standing in the corner of the room in the shadows with her back turned. Lief apologizes for taking so long but when Mrs. Creecher turned around to see them, Lief, Komodo, Guy, Dennis, Strawberry, Pieface, Curly, Yawp and Dander were shocked with fear when they saw that their teacher turned into a zombie!
Quickly, they ran out of school and escaped the zombie teacher. Outside, they ran into Sergeant Slipper who was looking at the strange patches of fog. Strawberry asks what's going on and how their teacher turned into a zombie, but Slipper had no idea why or how everyone is turning into zombies. Then he explains about the green fog that's appearing out of nowhere and shows them what happens if someone breathes in the fog by inhaling it through his mouth.
Realizing that the fog was the cause of the zombie transformations, Lief tries to stop Sergent Slipper but it was already too late! Sergeant Slipper had already turned into a zombie! The gang quickly ran away from him as fast as they could and luckily they got as far away from that zombie policeman as possible.
As they made their escape, Dennis whistles to call his dog Gnasher just to know if he's not a zombie. Just then, Gnasher came in time for Dennis' plan. He says to stop a zombie is to destroy their brains with chainsaws, light sabers, swords, and cool axes but Lief disagrees with that plan because first of all, they can't risk hurting the citizens of Hub City and second, they're not dead!
Just then, Curly had an idea! He pulls out his iPad and looks into a website called Zombiepedia to look at zombie types: fast, slow, undead, hypnotized, brainwashed, mutant, cursed, pirate cursed, secret experiment gone wrong and other types. Some zombie types can be cured. Lief finds it very interesting and Komodo is fascinated as usual, but Dennis finds it lame. Lief then tells his friend that it might be worth a try, even if it might be lame while Strawberry reckons that they should find out where the fog is coming from and Guy says that everyone doesn't call the Hub Squad the best mystery-solving team in Hub City for nothing.
Suddenly, a zombie unexpectedly appears behind them, ready to attack. Guy quickly transforms in SheZow just in time and kicks the zombie in the chest! Then Dennis punches it in the face while Lief both kicks and punches it in the back and butt!
After the zombie was knocked out, the gang quickly ran to get to Lief's castle where they'll go to a treehouse which Guy never knew about and asked why Lief had a treehouse at his castle just like Dennis. Lief explains that it's a long story as they ran just around the corner. Just then, they stopped as they saw the green fog heading straight for them!
Lief looked down and saw their way to escape; the sewer vent! So with SheZow's help, he and Lief pulled the lid off quickly and all ten of the friends went down into it before the fog could get them. Later, down in the sewers, Strawberry wonders how they'll get to the castle now that they are in the sewers so Guy pulls out his iPhone to use the GPS tracking app and leads the gang to the castle's location.
Dander complained that they should've used to bus instead because he thought that the fog stank more than the sewers but Strawberry relaxed him a little as they arrived their location when Lief saw a light up in the city. As he looked from left to right to see if there is any fog or zombies around, the coast was clear and the gang went to the royal gardens. But then they stopped as they saw more green fog.
Suddenly, Lief had an idea! They can get to the tree house from his bedroom, but the only problem is that they'll have to go through the castle first.
Inside, they saw Barda who was looking so green and moaning like a zombie. Lief tries to calm his friends down and tell them that Barda always looks like that in the morning but then he realized that it isn't the usual mornings because... Barda really is a zombie because he must've breathed in fog this morning! As fast as they could, they ran upstairs to the sorcery room to hide until it's safe for them to get to Lief's room and into the treehouse!
When they slammed the door shut, a hand landed on Lief's shoulder and he jumped and screamed. But it was only Jasmine, Filli, and Kree who were also hiding from Zombie Barda. Lief thought that Jasmine turned into a zombie to which annoyed her a little bit, just when he asked her why she wasn't at school.
Jasmine explains then that when she was in history class, she saw the green fog outside of the school which reached up from and opened window to Minerva who then turned into a zombie and the other students as well. They tried to get her, but Jasmine managed to escape the classroom without breathing in any green fog, then she went to the nurse's office to ask for some surgery masks to protect herself and her friends but when she got there, she found that the nurse was turned into a zombie too! So reluctantly, Jasmine jumped over her and grabbed the masks, then put one on, and escaped through an open window while Kree with Filli on his back flew to the castle.
Lief thinks that they should get to the treehouse right away to be safe from the zombies and make a plan. He asks Dennis and Gnasher to check if Zombie Barda is still out there. So they peeked through the keyhole on the door to check. Luckily the coast was clear and the gang quietly opened the door and tiptoed to Lief's bedroom and Jasmine handed them the masks that she took from the nurse's office. Then when they made it to the room, they hopped out from the window and onto a cart connected to a pulley, then Lief pulled on to the rope.
When the ten friends made it to the treehouse, Guy was looking through a spyglass watching all of the zombie fog and zombies roaming around while Lief was pacing back and front to try and make a plan to turned everyone back to normal while his friends were sitting in bean bag chairs, eating snacks. Suddenly, something caught Guy's eyes! He calls to his friends to see where the zombie fog was coming from, which was actually at a factory that's was a few miles away.
So the Hub Squad had another mission: Get to the factory and stop the zombie fog. Then find the zombie cure and return everyone back to normal! But there was just one problem: how will they get to the factory with all that fog and zombies roaming around?
Just then, zombies everywhere popped out from the windows unexpectedly and Lief and Dennis hit them with foamy bats and threw rubber bouncy balls at them. Then they used water guns loaded with tomato paste. But there were so many zombies that they cornered the friends.
Once again, the friends were lucky when Yawp hit the emergency escape button and all ten of them fell into a trapdoor that lead them back into the sewers. When they got up onto their feet, the Hub Squad taps their gems on their bracelets while Guy uses once again his GPS app and then finds the location to the factory's location. The gang follows him, trying not to slip on any slime puddles on the way.
- This episode is based on the new Dennis and Gnasher app game.
Lief: Quick! We gotta get to the castle treehouse! We'll be safe up there and make a plan.
Guy: What? The castle has a treehouse now? How come you never told me about that, Lief?