Mannix and Art Malcolm are being held by a unstable, psychopathic former member of their GI Unit who has killed the other soldiers who served with them, over a fourteen year period. It turns out he was a traitor to the whole outfit, and the unit (including Joe and Art) abandoned him during the war and he was imprisoned in Leavenworth. Now he wants to eliminate both of them since they know who he is and what he has done, however he has clearly underestimated Mannix's loyalty to his friend
—charmardee-smith