Chase was indisputably the leader of the PAW Patrol other than Ryder, this meant that Chase was the leader pup, as Ryder was not a pup. He wasn't as good as Ryder, this is known due to the few times he's had to lead the pups. He did it, but Ryder would've done it better, outside of Ultimate Rescues of course. All the pups knew Chase was most fit to lead, and they all accepted it as fact.
"Aren't rescues fun everybody?" Skye asked the pups after saving Cap'n Turbot for what felt like the seven hundredth time.
"They always are!" Marshall answered, "Our reward is always fun as well."
"Okay Marshall, you all can have some treats for a job well done." Ryder laughed, giving all the pups a treat one by one.
"I'm not hungry Ryder." Chase said, looking at his pup treat.
"What's wrong Chase?" Ryder questioned, noticing Chase looked unhappy.
"I don't know, I just... Don't feel well." Chase replied, looking now at Ryder.
"I can check your temperature." Marshall suggested, to which Chase and Ryder agreed.
As Marshall went to get his EMT gear, the pups were all worried for Chase. He'd been sick before, but it was always scary when another pup felt bad. There are lots of things that "I don't feel well" could mean, so nobody ever felt good when a pup said that.
"Alright Chase, open up!" Marshall commanded after getting his EMT gear and thermometer out.
Chase obliged as Marshall took his temperature. Interestingly, Chase was at a very average temperature, a little over, but nothing that indicated sickness. Marshall suggested that Chase go to his pup house and rest for a bit and see if it'd blow over, as Marshall couldn't find anything wrong with Chase.
"Well, pups, I'm sure Chase will be okay, so go have some fun!" Ryder told all the pups, to which they all listened.
Hearing both Marshall and Ryder say that Chase was fine made all the pups feel better. Sure he wasn't feeling well which made them unhappy, but they were glad to figure out it wasn't anything too serious. That they knew of anyway.
Really, Chase had no idea what was happening either. He was happy he wasn't required for the rescue to save Cap'n Turbot that day, because he'd been feeling some weird feeling in his chest. It was pain, but not a normal kind of pain like he just got punched in the chest, it was more like Ryder had just told him he had no use to the PAW Patrol and should leave. Ryder never said that though, never would. Chase's chest, more or less heart, just didn't feel right and he didn't know why.
"You okay Chase?" Rocky asked, startling Chase a small amount after Rocky had noticed Chase had his face buried in his paws.
"No. I still feel bad, and I have been feeling this way all day." Chase responded, lifting his face out of his paws to look at Rocky.
"Should I tell Marshall anything?" Rocky asked, concerned about Chase.
"No. I'll see if it's better when I wake up." Chase answered, unsure if it would be.
"Alright Chase, make sure to tell us tomorrow how you feel." Rocky told him, ready to walk away.
"I will Rocky... Thanks for being concerned." Chase said to Rocky, who said you're welcome and walked away to go play with the other pups.
When night fell and all the pups went to their pup houses, Chase was already asleep, ready for the next day to fix his problem. It didn't. In fact, the next day was worse, not unbearably worse, but worse. He'd honored his promise to Rocky and told the pups this, who didn't know what to do since Marshall still couldn't figure out the problem. This continued happening until the pain in his chest got to the pain of never leaving his pup house. The pups days had a lot of concern for Chase in them, day after day he got worse and Marshall didn't know what was happening, all he knew was that Chase's body was fine, and he wasn't dying at all.
After a while, Chase began getting moody and snappy towards everybody, which only served to concern them all even more. Chase wouldn't let Marshall check up on him, and even Skye got snapped at and told to leave. It was a real scary few days, and little did they know those days would be the least scary in recent memory.
Eventually, they'd figure out what was wrong with Chase. Eventually, the pain in Chase's heart would stop. Eventually, Chase would come out of his pup house. Eventually, Chase would become himself again. Atleast, himself in his mind, his changed, not the same Chase, mind. He didn't look any different, but now, as they'd soon figure out, Chase had no goals but to terrorize the pups whenever opportunity arose. Chase had changed. Chase, had become a monster.
Once it got dark, all the pups went to bed as usual, Chase's health in there mind as well. Rocky, though, didn't have a good sleep, as he was awoken in the middle of the night by a loud bang on his pup house. His eyes shot open, only to see nothing but white sharp teeth and a yellow police badge identical to the one on Chase's pup tag below them.
"Chase?" Rocky asked into the darkness.
The response Rocky got was getting shot in the face by a tennis ball cannon and the teeth and badge being gone after he recovered. This was the very first thing Chase would do, and it was nothing close to what he had planned. Rocky woke Ryder up to tell him he thought he saw Chase, which caused Ryder and Rocky to check Chase's pup house and sure enough; no Chase.
"Looking for me?" A more evil sounding version of Chase's voice would say from behind them.
"Chase? Is that you?" Ryder questioned, turning around and seeing the police shepherd standing there.
Chase's response was biting Ryder's leg, causing Ryder to kneel over, and then running away. Ryder yelled for Chase but Chase only ran the opposite direction no matter what Ryder said. Ryder told Rocky to get Marshall who hadn't left his EMT gear since Chase's condition got bad. After Marshall had patched Ryder up Ryder tried to call Chase, but got no answer. So, he called all the pups, waking them up and telling them to get to the lookout. Marshall actually didn't crash, because he was already in with Rocky and Ryder as the other pups entered the elevator.
"Uh, PAW Patrol ready for action Ryder sir!" Marshall called out as Chase wasn't there to do so.
"Alright pups, good news, and very bad news. Good news is Chase is out of his pup house. Bad news is... well... he seems to be against us." Ryder tried to explain, but he didn't know exactly what was happening either.
"What? Chase would never do that!" Rubble yelled after all the pups gasped.
"I know, that's why we need to figure out what's wrong with him." Ryder explained, "But, we need to watch out for him. We don't know what he might do."
"This will be different from a usual rescue mission, so all paws on deck to figuring out what's wrong with Chase, and fixing it." Ryder said, the big screen showing a little image of all the pups surrounding a happy looking Chase.
"It's best we wait for daylight so we can better see what Chase is up to. You should all stay up here tonight, and I'll keep the elevator up here so Chase can't get up." Ryder told the pups, "This is not a rescue where you will be going down the slide. Stay up here for tonight."
Ryder being as serious as he was was scary. The scariest thing about that though, other than the obvious Chase may have turned on them, was that Ryder didn't say "PAW Patrol is on a roll!" He only told them to stay in the lookout with a serious face and concerned tone. This was a serious mission, more serious than anything they'd ever done to protect another pup.
It just had to be the lead pup, didn't it?