The Tracy's in space! When the Hood threatens the craft Zero X, International Rescue agrees to help. Smart work by Lady Penelope and her chauffeur, Parker, foils the Hood's second sabotage attempt. Zero X reaches Mars to face new and terrible dangers, fire-breathing 'rock-snakes'. Badly damaged can Zero X avoid a crash? There is only one last chance!

All the Tracy boys — Scott, Virgil, John, Gordon and Alan are involved. Their courage and resource, the genius of Brains, the quick thinking of Lady Penelope, the scientific miracles of the Thunderbirds' equipment are all tested to the full in this thrilling new story.