The experimental North American X-15 program at Edwards Air Force Base involves test pilots: civilian Matt Powell (David McLean), Lt. Col. Lee Brandon (Charles Bronson) and Maj. Ernest Wilde (Ralph Taeger). The cutting edge high-speed program is ramrodded by project chief Tom Deparma (James Gregory) and US Air Force Col. Craig Brewster (Kenneth Tobey). As the test pilots prepare for the planned launch of the rocket-powered aircraft from a Boeing B-52 Stratofortress mother ship, they experience emotional and physical problems, which they share with their wives and sweethearts. Test after test results in setbacks, including a near disaster when an engine explodes during a ground test and engulfs the X-15 and its pilot in flames, but finally the X-15 begins to set records in speed and altitude for a piloted aircraft. When the X-15 "flames out" on a high altitude run, after guiding the X-15 to a safe landing, saving Powell's life, Lt. Col. Brandon, flying a chase aircraft, is killed in a crash. Powell himself takes the X-15 into outer space for the final test.