Help the secret agent select the grammatically correct travel itinerary for her next mission.

The correct answer is I will fly from the United Kingdom to Japan.

Most countries are singular proper nouns and do not take an article (e.g., Japan, France, Brazil). However, countries whose names include words like "Kingdom," "States," or "Republic," or countries with plural names, require the definite article "the" (e.g., the United Kingdom, the United States, the Philippines).