The series title says it all: The Cat in the Hat Knows a Lot About That!^TM But he doesn't know everything, which is what makes him the ideal guide to the natural world. His own keen sense of fun and curiosity is so contagious it sparks the kids to make their own discoveries. With panache and flair, he is like a subtle magician who helps reveal all of nature's tricks in response to clues detected or questions posed by the kids. He helps them explore their misconceptions so that, through trial and error, they come up with the logical conclusions. Each time he visits, he takes the kids on a roller coaster ride in his magical vehicle, the Thinga-ma-jigger, and delivers them back home, safe and sound and a little bit better informed. The Cat is smart, impish, funny, playful, suave and, often as not, lovably goofy.

Sally is Nick's best friend and next-door neighbor. A picket fence divides their back yards. Separating the fence is a big old maple tree in which there is a tree house from which many of their adventures begin. More analytical than Nick, she is a sounding board for his fantastical ideas. Where Nick is prone to flights of fantasy, Sally is more practical. When Sally's garden needs water, Nick says they need a giant hose with 30 nozzles, whereas Sally says they just need some rain. Fond of playing dress-up and doing arts and crafts, Sally also loves animals and animals love her.

Nick is infinitely good-natured and game to try almost anything and, if he winds up getting tangled up into a knot, he is the first to laugh at himself and the situation. Impulsive and instinctive, he flushes out problems and tests theories, jumping first and asking questions later. Nick loves the Thinga-ma-jigger and it is he who gets to flip the jigger-ma-whizzer that launches them on each adventure.

The Fish generally knows the answer to the kids' questions or problems but the Cat always interrupts him before he can get a word out. His worrying and warnings help intensify and heighten the sense of adventure, but he never spoils the journey. The Fish belongs to the Cat, and his bowl is generally a fixture on the Thinga-ma-jigger.

Thing One and Thing Two love to help the Cat and the kids solve a problem or illustrate a point with a lot of enthusiasm and a bottomless trunk full of outrageous gizmos. They live in the back of the Thinga-ma-jigger and pop out when needed like a couple of puppies let out to play. They make themselves understood with their acrobatic slapstick routines and pantomimes and communicate in non-verbal, musical language known as Thingese.

The Thinga-ma-jigger is the most fantastical flying, walking, swimming, sailing machine ever invented. It can go to any place or space - large or small, growing and shrinking to fit the size of any adventure. This magical contraption can sprout anything and everything from wings to booster rockets, to balloons, to skis - just pull the "right" lever and it pops out of a hatch.