Sarah Jane is in a Christmassy mood, but her cosy afternoon listening to carols is interrupted when Mr Smith suddenly announces that danger is imminent. The supercomputer has detected abnormal internet activity in the vicinity of Bannerman Road. Something alien is downloading into two PCs on the street — a computer virus over fifty times more destructive than any other on the planet.

Both residents were chatting online when they started the download: but who were they chatting to? And what does the mysterious chatterbox want? It is up to Sarah and friends to find out, before the deadly download takes over the world...