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"H" has just finished his final training with defence computers and due to the excellence of his record, his promotion to Major is assured. But before taking up his new duties he is entitled to three weeks' leave. Calling him into his office, the Colonel tells him that the last part of his training programme entails a visit to certain underground installations which will necessitate postponing his leave by a day or two. Just a day or two. But what H does not realise is that once underground, he is there for keeps.
H finds himself on "Level 7", 4,500 feet below the Earth's surface. Above him are six other levels, each one for a different population category: Government, scientists, service chiefs, etc. Each level is entirely self-sufficient even manufacturing its own air. Each inhabitant has been carefully chosen for his lack of family ties and responsibilities, and they will live in perfect safety. H will be known as X127, and he will work closely in the Operations room with X117, in case of an emergency they must both push the buttons together: the buttons that will send the deadly nuclear rockets over enemy territory. Each button controls a different type of rocket.
For a time life proceeds smoothly, except for isolated incidents. H's duty companion X117 cracks up and has to be given medical attention - medical attention which erases all memory of surface life. H himself falls in love and is allowed to get married. But he nearly find himself in serious trouble when he breaks into the plant room to give his fiancee a flower. They are denied access to the Married Room for one month.
(Suddenly, almost without warning, the final alarm goes. The enemy have let off their own rockets by mistake but it is too late to bring them back. H and his colleague, the new X117, have pressed the final button, and the evil dark stain spreads across the Operations Map. The War is all over in 45 minutes. The TV screens show nothing but ugliness and barren desolation left. Peace negotiations have nothing to negotiate about.
But Level 7 hasn't taken into account the deadly radiation poisoning which seeps down through the ground. Soon it has engulfed all the other levels, bringing with it blindness, vomiting and death. H and his wife die in each other's arms. The whole of Level 7 has been in vain.)