The show begins with a bully trying to take Felix's money. Hayley intervenes, and she and Felix walk away. As soon as they leave, however, the bully is attacked by a swarm of insects. He later returns all the money he's ever stolen from Felix. The swarm monster attacks more and more people, from cheerleaders to lunch ladies, until pretty much everyone except the Troop members have been attacked. The cheerleader who Felix has a crush on stops ignoring him and treats him affectionately. The Troop members take her back to HQ for a brain scan and find the insects in her brain. Jake doesn't understand why they've got to fight monsters that make everybody happy and well-behaved, but Hayley replies, "The SWARM is a monster. We fight monsters." Later, though, they have a better reason to fight the insects - the victims' brains start corroding and melting into purple goo.
It's not my favorite Troop episode because, frankly, I think that the idea of everyone acting pleasant is a great one; however, when everyone's brains start corroding, it was definitely time for the Troop to go after the SWARM. But I do love how The Troop can come up with brand new, creative, unused monsters. The SWARM is brand new - I just wish they had a less pleasant function.