It's the near future. Computers are faster, cars run on fuel cells, and there's not much to do in the small town of Easter River if you're thirteen and not into team sports. Helen Chan-Fisher is happy enough trying singlehandedly to save the world's remaining amphibians from chytridiomycosis, but her friend Jordan O'Blenis is at a loose end. He may be a genius, but it seems like no matter what he tries to do, his robot-building older sister Cassie did it first. Then he has his great idea, an idea so great even Cassie hasn't done it. He'll write a programme for a virtual supercomputer, one that can live on the Web, and grow and spread and learn .... Jordan calls it Cassandra. Helen calls it a virus. Cassandra calls home...and when agents of the government security agency Bureau 6 try to seize her for their own purposes, it's up to Jordan and Helen to keep Cassandra from falling into the wrong hands.

Something is cutting off Spohrville's communication with the outside world. The phones don't work. There's no radio, no TV—no internet. Are eco-terrorists trying to shut down the Mars Relay satellite? That's what the government says, but Jordan and Helen and the sentient virtual supercomputer Cassandra don't believe a word of it. The town is overrun with "birdwatchers" who can't tell a hawk from a heron. Jordan's old enemy, Harvey Number Two of the spy agency Bureau 6, is sneaking around pretending to be a cop on holiday. And archaeologist Uncle William has dug up a very strange black rock while excavating an Acadian settlement. With no land-lines to the site of the dig and wireless communication impossible, Jordan and Helen have no back-up from Cassandra. They've taken on corrupt government agents and industrial spies before, but they've always had Cassandra behind them. It's the twenty-first century. The bad guys have night-vision goggles and interference triangulators. How did Jordan and Helen get stuck with a bunch of musket-toting historical re-enactors as their only allies?