S1·E15 Flight Path

A SHADO Technician, who is being blackmailed for information by an unknown party, is caught by Freeman, but not before the information is passed. The Technician, Paul Roper, divulges what information he gave to the blackmailers, to SHADO. After much work on the cryptic data, they discover it is part of a plan by the aliens to attack the Moonbase in a way that would be undetectable. Now it's up to Roper to redeem himself, as he is sent on the suicide mission to stop the attack. If he fails, the base will surely be destroyed.