by Mordecai Roshwald.
Dramatised by J.B. Priestley.
Starring Anthony Bate, Keith Buckley, Michele Dotrice
and David Collings, Tom Criddle, Jane Jordan Rogers
(First shown on BBC-2)

After his training on defence computers, 'H' (X.127) is entitled to three weeks' leave. But his Colonel tells him that he must visit certain underground installations first, and his leave is postponed. So 'H' goes down to Level 7 - 4,500 feet below the earth's surface...