Nick gets some new train cars for his train set and wants to add them to his train, but the magnets won't connect. The Cat takes them to the Magnetic Fields where they learn how magnets work.Nick is trying to learn how to ride his bike so the Cat takes him and Sally to Balence Point, where he discovers the things to keep in mind when trying to maintain your balance.

Nick and Sally need a strong piece of string to pull their train, but it breaks, and they don't have anything stronger in their craft box. The Cat takes them away, but the Thinga-ma-jigger malfunctions and they have to make an unscheduled landing. It turns out they've shrunken and are inside the craft box, where they discover that they can create a stronger string by braiding several strands together.Nick and Sally are in the treehouse making funny faces at themselves in the mirror when they notice a bird flying back and forth into the tree branches. They discover that the bird is building a nest and they want to get a closer look but can't do it without disturbing the birds. The Cat takes them to a mirror world where they learn how reflections work and they concoct a kind of periscope that allows them to watch the bird without disturbing it.

The kids need to move a big rock in their backyard so they can plant an apple tree. The Cat takes them to Machine-a-Ma-Zoo, where they learn how a lever can solve their problem.Sally is playing a song on an instrument made up of different things in the Treehouse but she can't find the right note for her song. In Gongolia, they find out that bigger things make lower sounds and smaller things make higher sounds.

Sally is trying to make a vase for her mother but each one she tries falls over. Cat takes the kids to Blueprintia where designs become real. They discover what they can learn by designing before building, which helps Sally make the vase back at home.The kids are creating a game that involves "fishing" for prizes, but they can't "catch" any of the prizes. The Cat takes them to Arcade Island where they discover what will and won't be attracted to a magnet.

Nick and Sally are having a sleepover in the treehouse and Nick is startled by a shadow. The Cat explains that shadows aren't scary if you know what they are. He takes them to Silhouette Valley where they learn that shadows are made by blocking light and the position of the light can make shadows grow or shrink.Nick unintentionally loses Sally's bouncy ball. They go to Ballaballo and learn how different balls are made of different materials, which help them move in different ways.

The kids want to get a trunk up into the treehouse and go to the Machine-a-Ma-Zoo, where they're introduced to simple machines, and learn about the pulley.Sally loses her "slidiness" and when they go to the Frictionarium, they find out that there's a force called friction that can stop a person from sliding.

Nick wants to create a new home for the cricket in their backyard, but the cricket won't go in. The Cat takes them on a journey but has to make an emergency landing on Lagoona-Maroon, where they discover what they really need to consider when making a shelter.Nick and Sally are frustrated by a puddle that always forms at an inconvenient spot on the sidewalk. They go to a water park and learn that water always seeks the lowest level.

Show Me the Honey - Sally and Nick have run out of honey! Luckily the Cat knows just where to go to get more. They visit Queen Priscilla Buzzoo who invites them to join in the bee dance to learn how to make honey.Migration Vacation - The purple martin swallow that lives in Sally's backyard has gone missing! The Cat, with the help of a variety of forest creatures, help Nick and Sally track down the bird, and learn how and why birds migrate.

Sally wants to fly her kite but can't seem to work out the timing to get the wind right - after all, you can't see wind. The Cat takes them to Windnasium where they learn to "see" wind in all kinds of ways.When Nick's favorite flyer gets stuck in the tree, Nick wishes he could jump really high and retrieve it, but gravity holds him down. Cat takes them to Gravity Drop where they discover what happens when there's more or less gravity. And Nick figures out how to use gravity to get his plane back.