Twelve-year-old, David Griffin, is a baseball fanatic and a sixth-grade student. The weather in coastal Connecticut is unseasonably warm at the beginning of March, so David is distracted by his daydreams of being a star baseball pitcher. But then David learns that he has to pass Monday's test on weather in order to pass science and live out his dream. David's parents are doctors and his Grandpa is a weather scientist, so there will be no baseball season or season tickets to see his favorite baseball team play if David fails. David and his best friend, Jon, decide to search Grandpa's lab in the family basement for something to help him pass his test. In the process, they discover the Snowmallows - marshmallows shaped like snowflakes that were scientifically altered by Grandpa to make it snow when they are put into mugs of hot cocoa. Unfortunately, David and Jon don't read all the instructions on the bag and use too many Snowmallows. They set off a snowstorm that doesn't stop - where snow piles reach 6 feet high in some places.

David and Jon work with David's little sister, Emily, to stop the Snowmallows, but their solution, called Lemonthaw, causes a time shift. No one remembers the snow except the three of them. David fails science, is grounded, and will miss out on baseball season.

Grandpa's the only person left who can help David stop the Snomwallows. Unfortunately, Grandpa's the last person he wants to ask.