Capt. Jim Hollenbeck, a dedicated physician assigned to the space branch of the United States Air Force Medical Corps, voluntarily undergoes jump school training in order to better analyze the effect of parachute jumping on the human body. Upon his return from jump school, Jim is sent by Lt. Col. Masters, the head of the space program, to Sovern Air Force Base in Florida to investigate a problematic, experimental ejection seat. Test pilot Mike Bentley broke his shoulder while ejecting from his aircraft, and Jim is assigned to determine whether the problem lies in the equipment or in pilot error. Jim decides to replicate the jump himself, and when his arm is also broken, he announces that the wind blast during ejection forced his arm off the trigger, and hence, the problem is in the seat itself. Once the seat mechanism is corrected, it must be tested at supersonic speeds, and so a series of experiments are begun with rocket sleds traversing the ground at 1,000 miles per hour. Dr. Hugh Thornton, an advocate of the program and Jim's mentor, dreams of one day launching a platform into the upper reaches of the atmosphere to test the effect of high altitudes on man. To accomplish this, Thornton asks Jim to test the first balloon gondola designed to carry a man twenty miles up into the "threshold of space," the first step toward putting a man into space. Before accepting the assignment, Jim confers with his fiancée, Pat Lange, who also works in the Air Force program. After Pat gives Jim her blessing, the two are wed, but their honeymoon is cut short by the news that Masters has been killed in an automobile accident. Jim is summoned back to the base, where he meets Maj. Ward Thomas, the new head of the program, who is more circumspect than his predecessor about opening up the frontiers of space. Consequently, the major cancels Jim's balloon flight until the effects of high altitude on the human body can be better understood. To this end, the major assigns Jim to pilot Lt. Morton Glenn in the balloon gondola to a height of 55,000 feet, after which Jim is to lower the gondola to 10,000 feet, where Glenn will parachute to earth. Meanwhile, tests proceed on the rocket sleds with crash dummies. After a dummy is decapitated, the major gives orders for a live jump from the gondola. As the gondola descends from 55,000 to 10,000 feet, Glenn freezes, afraid to jump. When the gondola then begins to drift past its target, Jim decides to jump himself, defying the major's orders. Although Jim parachutes safely and Glenn is only slightly injured when the gondola crashes to earth, the major grounds Jim for insubordination. Jim accuses his superior of opposing all human experimentation until he learns that the major plans to test the sled himself and has assigned Jim to act as physician for the trial. After attaining speeds up to 1,000 miles per hour, the major is temporarily blinded, but soon regains his sight, and the test is proclaimed a success. When Washington authorizes a floating high-altitude platform, Jim, chastened, apologizes to the major for his rash behavior and offers his resignation, which the major promptly tears up. Jim is then assigned to the first high-altitude balloon flight, and as he ascends 100,000 feet into the atmosphere, Pat, Thornton and the major watch apprehensively from earth. Jim's radio fails, and tense minutes pass until he is able to make repairs and regain contact. As Jim drifts over a rugged mountain range, he discovers that his oxygen is nearly depleted, forcing him to land. While descending into the peril below, Jim unemotionally reports that his mind is not operating at peak capacity and that he feels a total separation from earth. As Thornton and the major listen to what may be Jim's last words, they admire his fortitude and analytical abilities. After the gondola crashes in the mountains, a helicopter locates the craft and finds Jim alive, but dazed.