Most people are aware that the original Adventure Island was in fact a slightly modified NES port of Wonder Boy for the Master System. Immediately after this, Wonder Boy became more of an action RPG-like series, while Adventure Island stayed a platforming series.
That is, until the release of Adventure Island IV, which sadly stayed Japan-exclusive. Surprisingly, this one followed the same route Wonder Boy had taken and was also an Action RPG. They decided to keep this formula for the next game in the series, Super Adventure Island II, which would also be the last game for a long time.
In a series first, this time, Higgins and Tina, his girlfriend, have actually finally married; they're actually on their honeymoon. However, they're suddenly ripped apart by a storm, and both lose their memory, so Higgins now has to both find Tina and get his and her memories back. He washes up on Waku-Waku Island, where the king tells him that his bride-to-be was kidnapped, so Higgins, of course, decides to go rescue her and possibly find some clues for his own quest along the way.
The king's future wife was taken to the nearby Fuwa-Fuwa Island, but there's no way to get in there at the moment, as it's protected by a barrier which can only be broken by collecting five spells from the nearby islands. Not one to give up so easily, Higgins goes to visit every island one by one to defeat their guardians and get the spells.
As stated, this Adventure Island plays very differently from the previous games. If you open the pause menu you'll quickly discover that Higgins doesn't just get different weapons, but he'll also find suits of armour, shields and even magic spells during his travels. Each island is basically a self-contained "dungeon," unlike the giant connected world in Wonder Boy in Monster World. Each has a few simple puzzles to solve and treasure chests to find, but nothing's really hidden: everything can be seen out in the open.
However, as you get further into the game you'll find that there is a small degree of interactivity between the islands after all. Each has a warp to two other islands (Basically creating an entire circle of warps), as well as its own unique switch which removes or places several blocks bearing the same insignia on all of the islands. You won't be done with the first island as soon as you beat the boss, for example: you should still come back later after you've activated some blocks you couldn't jump on before, or when you've found a shovel later on that lets you dig through blocks of dirt.
The islands are pretty small so you shouldn't get lost at all, but if, somehow, you do, then there's a map available on the pause screen, as well as a magic spell that returns you to the island's entrance. Of course, magic uses up magic power from a bar displayed at the top of the screen, but there are many extensions for it you can find to increase the amount of spells you can cast before depleting it all. Naturally, the same goes for health.
While at the start of the game Higgins will mostly be using swords, later on, you'll find some more diverse weaponry, like a set of rotating fireballs, an axe and a boomerang, all from past Adventure Island games. There's also a casino where you can play slots, roulette or bet on monster races, with the winnings allowing you to buy the best equipment in the game. It's absolutely unnecessary to beat the game, but if you do find yourself stuck at some point, maybe you could invest some time into it.
Previous games in the series never were really impressive graphics-wise, but this time everything just looks slightly low-budget for some reason, with all the backgrounds being fairly basic and not very detailed, and screen transitions being handled by a quick jump to a black screen for no apparent reason. The music, while not as good as Yuzo Koshiro's efforts for Super Adventure Island, is still very catchy and can get stuck in your head for a long time.
It's a radical gameplay change, but perhaps for the best — Super Adventure Island II is arguably the best game in the entire series, and well worth a play even if you're not a fan of the previous games. It strikes a perfect balance between being easy for newcomers and challenging for veterans, ensuring that no matter how good you are you'll have a good time.