The Avengers (1961)

S5E4 The See-Through Man

An invisible intruder has breached the Ministry, leaving everyone puzzled. The investigation leads to inventor Quilby, who connects to the Eastern Drug Company, a cover operation for Soviet intelligence. At the same time, skilled spies Elena and Alexandre Vazin are in the area, but there's a mystery surrounding Alexandre's whereabouts, as no one seems to have spotted him.