Each episode in this series featured a lengthy interrogation by Colonel Pinto ( played expertly by Bernard Archer) to test whether an apparently genuine Allied serviceman or refugee was, in reality, a German spy. His ingenious methods invariably resulted in the suspect giving himself away. Despite the inevitability of the outcome, the series was a fascinating study of psychological tension. Like the later detective series `Columbo', this was not so much a `who-dun-it?' as a `how-will-he-do-it?'. The series was based on real life - Colonel Pinto was a Dutch intelligence officer working in Britain during the war who was generally regarded as the best in his field. Despite the sparse set (there was just one room, with a table and two chairs), and the small cast, this was an excellent and memorable series.