This book introduces Group Captain Timothy "Tiger" Clinton, R.A.F. (retired) and his son, Rex Clinton, while they are spending a holiday in Inverness-shire. Lost in the Scottish fog, they see red lights and follow them to lonely Glensalich Castle, where they meet Professor Lucius Brane and his butler, Judkins. The Professor has heard of Tiger Clinton as he is a well-known aircraft engineer, and as a result shows him a vehicle he has been building for space travel: the Spacemaster. The vehicle is still being tested. The Professor has managed to harness the powers of cosmic rays, which are everywhere, as motive power. Initial tests are by remote control, but then the Professor, with Tiger, Rex, and Judkins, goes into orbit for the first time.