The story is based on the novel "Boku wa Okinawa wo torimodoshitai Isshoku no Gaikoukan Chiba Kazuo" by Miyagawa Tetsushi which features Chiba Kazuo who was one of the key persons negotiating the return of Okinawa to Japan after World War II. Iura plays Chiba who used to be a communications solder in the navy during the war and could only listen to the wireless communications of the US army as they attacked Okinawa. After the war, he became a diplomat and made a pledge to his wife Keiko (Toda) that he will get back Okinawa one day from the Americans who occupied it and used it as a base for the Vietnam war.
As Chiba was said to be so good at English that he was even better than the American natives, Iura had to prepare for the many negotiation scenes where English had to be spoken. He admitted that he actually doesn't know much English but in order to be convincing as Chiba, he underwent training by a teacher for about 6 weeks and tried to decipher the emotions which Chiba felt rather than just memorise lines. Even when at home alone, he would mumble in English as practice for the filming which he devoted himself fully into. Although he didn't think that he could become Chiba fully, he felt that he had expressed Chiba's strong mission in life as he fought on alone in that era. Iura also thanked Toda for being a good actress to work with as it helped him to interact with her naturally as a couple in the SP.
Sources: Oricon / Nikkansports