Being one of the launch titles for the N64, it basically serves as a tech demo for the console. The plot/setting is very low-stakes, as you are simply performing "flight tests" with different vehicles rather than fighting some villain. There are six pilots in total to choose from - three weight classes (light, medium or heavy) and a male and female character for each weight class. The light characters (Lark and Kiwi) are best with the Rocket Pack vehicle, the medium characters (Goose and Ibis) are best for the Hang Glider, and the heavy characters (Hawk and Robin) are best with the Gyrocopter. At the beginning, only the "Beginner Class" for each vehicle is unlocked, but if you get enough points for the mission in each class, you will unlock the next class of more difficult missions. The Beginner Class has only one mission for each vehicle, then the next class will have two missions, then three. Your point total includes all the missions for each class (so for example if you require 180 points to get the medal, you can get 90 points on mission 1 and 90 points on mission 2). Higher points will get you a silver medal, and if you get a perfect score, you will get the gold. Therefore, I would consider getting at least the bronze medal on each class to count as beating the game, and 100% completion would be getting the gold on every mission.
The missions take place on various islands, and there is also a "free flight" mode where you can simply explore the island. The missions are usually something like "fly through all the rings" or "hit all the targets." The gyrocopter has missiles that can be fired and the hang glider has a camera, so hang glider missions can include taking a picture. You get more points for finishing faster, getting a good/well-centered picture on camera missions, and for landing successfully (you can still finish the mission if you crash, but you will not get a good score). I don't think I am enjoying the game enough to bother finishing it, but I may play around with the free flight mode.