Complex sentence: The NKVD alleged it quickly uncovered the plot despite German secrecy following a tip-off from Soviet agent Nikolai Kuznetsov who was posing as Paul Siebert, an "Oberleutnant" in the "Wehrmacht" from Nazi-occupied Ukraine.
Split into simpler sentences: The NKVD alleged it quickly uncovered the plot despite German secrecy. It stated the first tip-off came from Soviet agent Nikolai Kuznetsov who was posing as Paul Siebert, an "Oberleutnant" in the "Wehrmacht" from Nazi-occupied Ukraine.