The ex-Stasi use a laser to listen in on Maria and Otto from across the street and through a closed window. An interferometry microphone using infrared light was built in 1947; when lasers came out that just made the device better. Sound waves make anything in a room vibrate microscopically; laser interferometry can read those vibrations and convert them into sound. In the 1980s this was very high tech.
This was originally broadcast three weeks after East and West Germany officially reunified, and one year after the Berlin Wall came down.
The Stasi was the secret police of Communist East Germany. It was the most extensive, aggressive, and oppressive domestic intelligence service in European history. They had insanely detailed dossiers on all dissidents and suspected dissidents, and merely impressive dossiers on everyone else. It is estimated that one in ten East German citizens had been bribed or extorted into secretly, routinely reporting on friends, neighbors, and relatives. After the wall came down a lot of people had to carefully hide their past involvement with the Stasi.