This is a fine basic plot overview, but if you really want to get the full backstory of how the original King's Quest came to be, I recommend this piece from The Digital Antiquarian:

http://www.filfre.net/2013/07/the-unmaking-and-remaking-of-s...

A note of warning - if you're of a certain age, be prepared to get sucked in. It's a massive trove of some of the best written and researched longform content on the history of PC gaming I've ever seen. Sierra, LucasArts, Infocom, Origin, Broderbund, Commodore, Atari... it's all there. Great stuff.