The book opens on the last day of school (before summer break) for Judy Moody and her friends. Her teacher, Mr. Todd, says he'll be working somewhere cold this summer and offers a prize to anyone who can find him. Judy is concerned that her summer will be boring and has a plan for herself and her three closest friends (Amy, Rocky and Frank) to make sure the summer isn't boring. She creates a game that involves a list of 'dares,' meaning something fun, but never done by that person before and is borderline frightening. Thrill points are earned by successfully completing the dare, and bonus points are added if the dare is done in an extreme way. Loser points are given if the dare is not completed. If the final score breaks 100 points, then summer was confirmed exciting. It's a fine plan, but Judy doesn't realize that two of the four of them (Rocky and Amy) will be out of town for the summer. Making matters worse, their own plans (circus camp and a trip to Borneo) sound far more exciting than hers.
Stuck at home with nothing more to do than read Nancy Drew adventures, Judy is not happy. It doesn't help when she learns that her parents will be traveling to California to help her grandparents move and that she and her brother, James (who goes by the nickname Stink), will be staying home from that trip, too. Their Aunt Opal is coming to watch them for the summer. Judy has not met Aunt Opal, but she assumes the worst. Aunt Opal, however, is surprisingly interesting — a free spirit who lives a life of adventure (including a stint in the Peace Corps, a trip across the Sahara Desert, and most recently, living in Bali).
Judy realizes that perhaps the four friends can still do the dare game, although they will need to keep track of their own points. To increase the stakes, Judy decides to make it a race, where they will be competing against each other. The first one to 100 points wins.
This begins a series of well-intended adventures designed to rack up points as quickly as possible, but every adventure goes horribly wrong. An attempt at tightrope walking across the local creek is threatened when Stink claims one of the trees to build a Bigfoot trap. Once Judy and Frank have the tree to themselves, Judy loses her balance crossing the rope and falls into the creek. A trip to the amusement park is ruined when Frank throws up on her. An art project with Aunt Opal is ruined when Judy accidentally glues her hand to the table. It takes several hours to get free. Going on a zombie walk through a graveyard at midnight sounds promising, and Aunt Opal even makes sandwiches for a picnic beforehand. Unfortunately, Aunt Opal hasn't driven a car for a long time; they quickly lose the map out the window and spend several hours careening around town. Eventually they end up stranded — out of gas and hopelessly lost outside an abandoned amusement park in the middle of the night. They go inside the park to eat their sandwiches, but their sandwiches had gotten too close to Stink's 'scat' collection in the kitchen and have manure samples on them. It's noticed before anyone eats them, but makes for a long, hungry night.
Another time they try going to a local horror movie festival (complete with monster costumes), but the scary zombie movie is too much for Frank, and he leaves. This is bad on several levels: Judy tries to force Frank to stay in the theater and ends up tearing his shirt. The two of them argue, and he complains that her obsession with quick points is taking all the fun out of the summer. And since they didn't stay through the movie, Judy can't even claim any thrill points for completing the dare.
Several times through all of these adventures gone wrong, Judy and Frank think they've found Mr. Todd's cold summer job, but they can't find him to win even that prize. Judy also keeps hearing about her friends' more successful summer adventures and the thrill points they are gaining. In turn, she can write to them only of her 'failures,' such as when she and Aunt Opal try to deliver their art projects (hats for the stone library lions) in the rain. Or when she and Frank try learning to surf, and Judy ends up kissing a dead jellyfish.
Finally, in desperation, Judy joins her brother in his own summer adventure: a quest to find Bigfoot. There have been numerous Bigfoot sightings in town throughout the summer, and her brother is convinced that not only is he real, but living nearby. Throughout most of the book, Judy has looked down on Stink's quest with big sister disdain. But that changes upon realizing that finding Bigfoot would mean lots of thrill and bonus points, and might actually let her win this race against her friends. So Judy jumps in with abandon, which includes Bigfoot Believers Association meetings, overnight stakeouts, bait traps and night vision goggles.
At first, it really does appear to be more of the same — one more promising adventure with repeated disappointments. At one stakeout, Judy and Stink 'capture' Frank (who looks considerably more like Bigfoot through Judy's night vision goggles, especially when he is tangled in their bait traps). Another time they clearly see Bigfoot and give a valiant, chaotic, and rather destructive chase through the neighborhood using bikes, a Vespa scooter and a van. Admittedly, it's weird when Bigfoot commandeers an ice cream truck, but even that makes sense by the end. (This particular sighting involves Zeke, a teenager who was asked to dress up as Bigfoot and come on a local ice cream route to help drum up business.) Zeke is not behind all the sightings though — he's one of the Bigfoot Believers Association leaders and quite convinced that Bigfoot is real and living somewhere in their neighborhood.
At this point in the story, things do begin to look up. Although no Bigfoot, they do find Mr. Todd at the end of the chase. He is the ice cream man who hired Zeke. And the prizes Mr. Todd gives to students who find him at his summer job are tickets to the circus, which is coming to town. This is the same circus that Judy's friend Rocky has been training for at circus camp. Judy gets an extra ticket for Frank and apologizes to him for letting an obsession with winning overshadow the summer. Judy's parents come home from California in time to see the circus. And at the circus, Rocky picks Judy as the audience volunteer for the act where they appear to saw someone in half.
Now that Judy and Stink's parents are home, Aunt Opal prepares to leave. Before going though, she sneaks Judy out to deliver the hats they made together — placing them quietly on the stone lions at the library. Although this does earn Judy 10 official thrill points, we get the sense that she is, in fact, grateful for the whole summer — even the adventures that didn't earn her points at the time. As they head back home, Judy sees a tall, shaggy figure disappearing into the brush. All Aunt Opal sees when she looks back are branches rustling in its wake, which confirms nothing (except that Judy did apparently see something tangible). When they arrive back at Judy's house, Stink has set up shop on the front lawn, selling tickets to 'touch Bigfoot.' This is not the mystery figure Judy just saw, but the art sculpture on their lawn that Aunt Opal made earlier in the story. Judy jumps into this opportunity, too, eager to earn money to go to France next summer with Aunt Opal.