Mike's Dad, an astronomer, is transferred from England to Tidbinbilla satellite tracking station, Canberra, and Mike comes too. He meets Ali and her sister Fiona, whose mother is a scientist at the station. Ali is a computer hacker who is breaking into the station's main computer, using an old satellite as a diary. She learns by accident that her Mum's boss, the horrible Dr Giles, is expecting a satellite to crash somewhere close by. The children want to be the ones to find it, but they don't know that on board the satellite is a deadly small pox virus, a secret experiment, and their adventure could mean risking death.