The mark of the pasha

The Great War has ended and Gareth Cadwallader Owen, who has spent his career defusing political time bombs, learns from his agents, some Greek and some Egyptian, that the streets of Cairo have been made dangerous by threats of real bombs. The first order of business is to ward them off. The second is to insure the safety of an impending major European delegation to the capital. What does it all have to do with Owen's shiny new motor car?