Spectrum are involved with the safe, quiet transportation of escorting the world's leading astrophysicist, Doctor Congrad, to a conference with the World President at Lake Toma in Switzerland.
After boarding their mysteriously-empty flight, which could possibly be due to it being the "slack" season with planes flying usually 3/4 empty but with no hostesses around, Captains Scarlet and Blue become suspicious, and realise that they are trapped on board the airliner, along with two tagging-along and over-confident news reporters. They soon learn that there aren't any pilots flying the machine, immediately suspecting the Mysterons are controlling it.
But that's not the least of their worries when the plane starts to descend rapidly, and its nose pointing straight in the direction of the French Alps.