Ian receives ventriloquists' dummy Slappy as a twelfth birthday gift, and when Ian's annoying cousins recite the magic words that bring Slappy to life, the doll unleashes pranks that are blamed on Ian.

As spring arrives on Gardam Street, there are surprises in store for each Penderwick, from neighbor Nick Geiger's expected return from the war to Batty's new dog-walking business, but her plans to use her profits to surprise her family on her eleventh birthday go astray.

Unhappy that his parents are expecting a baby and that his birthday party will be nothing special this year, fifth-grader Hank decides to celebrate by doing his favorite things, with only his new pet tarantula, Rosa, for company.

After celebrating their first nine same-day birthdays together, Amanda and Leo, having fallen out on their tenth and not speaking to each other for the last year, prepare to celebrate their eleventh birthday separately but peculiar things begin to happen as the day of their birthday begins to repeat itself over and over again.