The key character is General Bonner Fellers, tasked with investigating the role of the Japanese Emperor Hirohito in war crimes. The other key character is General Douglas MacArthur, who just happens to be one of my childhood heroes, played as Tommy Lee Jones by Tommy Lee Jones. I couldn’t quite wrap my head around the US marshal, being my MacArthur :P
While Fellers went around carrying on his investigation. He also engaged in a side quest searching for a Japanese woman that he met while studying in the US. The significance of this thread of the story is that because of his relationship with the Japanese woman and her family, Fellers had a good understanding of the often inscrutable Japanese culture which helped towards his final decision.
In the end, Fellers concluded that they couldn’t find any evidence that Hirohito is involved in war crimes. Likely because the fanatical devotion of officials (most of them are also going to be hanged) and their desire to protect the emperor. But the emperor has undisputedly called for an end to resistance and any further bloodshed. Hanging him would undo that. That’s what saved him.