This true story is set in Stalag Luft III—the same POW camp where the real events depicted in the film The Great Escape took place—and involved Williams, Michael Codner and Oliver Philpot, all inmates of the camp.  The prisoners were faced with the problem of digging an escape tunnel despite the accommodation huts, within which the tunnel entrance could be concealed, being a considerable distance from the perimeter fence.  They came up with an ingenious way of digging the tunnel with its entrance located in the middle of an open area relatively near the perimeter fence and using a vaulting horse (constructed largely from plywood from Canadian Red Cross parcels) to cover the entrance.

Eventually, as the tunnel lengthened, two men were hidden inside the horse while a larger group of men exercised, the two men continuing the tunnel digging.  At the end of the day they would again conceal the tunnel entrance and hide inside the horse while it was carried back to their hut.  They also had to devise a method of disposing of the earth coming out of the tunnel.  For the final breakout Codner hid in the tunnel during an Appel, before three men were carried over in the horse: the third to replace the tunnel trap.

The film was shot in a low-key style, fairly soon after the war, with a limited budget and a cast including many amateur actors.  It contributed to establishing the genre of British prisoner of war escape films.  Some details from Williams's book were not used in the film, e.g. the escaped POWs discussing the possibility of visiting potentially neutral "whorehouses" in Germany. (The idea was abandoned because of fear that it might be a trap, not out of prudishness.)