The story as I understood it is this:
The main character, Jim Hawkins, finds a map of the old pirate Captain Flint showing where he buried his treasure. Jim, a Doctor, a cook named Long John Silver, and a crew set off to find the island. Just before they reach their destination, most of the crew mutiny and it is revealed that Long John Silver is the captain of the mutineers.
On the island Jim finds a man called Ben Gunn (who would like more than anything to eat some cheese). Gunn teams up with Jim, the Doctor, the Captain and a few other loyal sailors. Jim goes on a stealth mission and somehow manages to sneak aboard the ship and move it so the pirates can't find it. He is captured by Silver, there is a hunt for the treasure but the good guys beat them to it. Jim and the rest of the good guys find the ship and sail off (taking Ben Gunn with them) leaving the mutineers behind to slowly go mad.
I'm sure it's an excellent story. If only it had more modern language or if it included footnotes to explain the words I didn't understand! I am going to give it a 46 out of 100 for being a good story but difficult language for modern kids to follow.