Fun Fact: StarTropics is one of the few NES games released in North America but not in Japan.
StarTropics is an adventure game with a twist: the hero is just an ordinary kid named Mike. Mike travels to a Caribbean-like island to visit his archeologist Uncle, but when Mike arrives his Uncle is missing. Armed only with a yo-yo, baseball bats, baseballs and his pitching arm, Mike sets out to find him. Overworld exploration that included seeking information from villagers revealed dungeon locations where the game's combat took place.
At a certain point in the game, players had to wet part of the game manual to reveal a code without which they could proceed no further. The code was the same for everyone, of course, but this was before gamers had the ridiculous amount of web resources for hint guides and cheat codes that they have today. If you lost your game manual and forgot what the number was, and had no friends who had the game or knew the number, or couldn't get the issue of Nintendo Power which published the number because so many people were asking for it, you were screwed.
It's 747, by the way. You're welcome.