Andrew and Evie aren't initially sure what to make of the package that arrives at their door soon after they move to a new town. It's filled with random things--a key, something that looks like a kaleidoscope, a seed packet, and a cryptic note. But over time, each item takes on unique importance.
The twins decide to fill the dull weeks before the start of school by spying on their neighbors. Their first target is their next-door-neighbor. At first she seems like a run-of-the-mill older lady, but she turns out to be a retired code specialist. She teaches them to create and break codes, skills that become important as twins' search for their mother intensifies.