Twins Willow and Jake Grimm are not prepared for the bizarre gadgets and scary changes in their new home of New Marburg, the location of the top-secret Think Tank where their parents now have jobs as scientists. In this town, people jetpack to work, robots direct traffic, and senior citizens duel with laser swords.

Yet nothing compares to what happens the day after lightning strikes the twins' house. Their school building transforms around them, becoming a medieval castle—complete with moat, drawbridge, and a dangerous giant who looks strangely like the school's overly aggressive hall monitor.

Barely managing to escape before a massive beanstalk lifts the castle above the clouds, Willow and Jake learn that their ancestors, the original Brothers Grimm, were more than storytellers. They were Grimmwalkers, able to travel from our universe to Grimmworld: the universe where all fairy tales dare born.

Somehow, Willow and Jake have been transported to Grimmworld as well. But before the twins can figure out how to get back to their own universe, they discover that their best friends, Hank and Pearl, have morphed into the fairy tale characters Hansel and Gretel and been captured by a candy-obsessed witch deep in the Cursed Forest.

Now, aided by a mysterious wizard with shimmering golden eyes and a befuddled, talking naked mole-rat named Chet, the twins must trust in each other, believe in themselves, and find the courage to face their darkest fears in order to save their friends, return to their family, and maybe—just maybe—save the world.