Complex sentence: Marvin explains that he and Frank are being hunted because they were listed as participants in a Clandestine operation codenamed Nightshade, conducted during the Cold War to smuggle a nuclear weapon into Russia piece by piece.
Split into simpler sentences: Marvin explains that he and Frank are being hunted down because they were listed as participants in a secret operation codenamed Nightshade. The operation was conducted during the Cold War era in order to smuggle a nuclear weapon into Russia piece by piece.