Marshall learns that he might need a retainer, which doesn't exactly thrill him because of something that happened a few months ago. A boy he knew from school got a retainer, and a few hours later, he started hearing what dogs think. As Marshall explains to his friend Simon, it's because the retainer tunes him into a different frequency.
Simon is less than thrilled with the idea that dogs can talk, but Marshall once again explains that away by pointing out that maybe they are speaking dogs. It doesn't help that the dogs clearly want to overthrow the world. They mention how much they hate humans and the way that humans treat each other. The trio of boys wind up in the pound, where the dogs give them a list of demands that humorously include no more leashes. Marshall and his friends end up saving the day, though their friend Steve disappears in the process.
The episode has a minor flaw that irritates me. Marshall mentions that the story with Steve happened in the past, and he actually remembers the events while sitting in the dentist's chair getting his own retainer. After coming home and showing his sister his new retainer, the first dog Steve heard talk suddenly appears in his driveway. Marshall flips out and then finds Steve's retainer stuck to the dog, and he acts like the story just happened.
It's still a cute little story though. There's a cute little poodle, complete with French accent, that wants to take over the world and gives their demands. It's also pretty funny to see the three boys wandering through a pound, listening to a bunch of random dogs talk about wanting to destroy all humans.