Don't let the title's lack of "B-words" fool you; Lunar Knights is basically a next-gen version of Boktai, the isometric vampire-stabbing adventure that used real sunlight to power your weaponry - but without the cartridge-based light sensor and all of the frustratingly limited game play issues that go along with it.
In place of said light sensor, Kojima Productions takes a more creative tack in Lunar Knights. While the playable characters still rely on sunlight (and now, moonlight) to power various weapons throughout the game, everything's based on the weather being shown on the DS' top screen. If the moon is shining and the clouds are light, you can draw power from it on the bottoms screen.
And in a really nice design twist, having any of the three GBA Boktai games in your DS' GBA cartridge slot allows you to use their sunlight sensor to use real-world light to augment Lunar Knights even more.
Gameplay is fairly straight forward - Lunar Knights' mechanics are almost identical to the previous Boktai games. Wander around each level looking for treasure, fighting bad guys, and searching for the next boss. Combat and movement are a tad sluggish, which is the game's biggest downfall. It becomes annoying to plod through each level.
Lunar Knights has a "dash" feature, but it'll cost you some energy if you want to run fast, which just sucks. The mechanics aren't as ridiculous as, say, Lunar: Dragon Song, but they're bad enough to keep most gamers from using the feature.
One of the worst bits about Boktai was when you finally defeated a boss and trapped it in a casket. You then had to drag said pine box back across the level in order to "purify" it. That whole segment's been excised from Lunar Knights. In its place is a slightly less annoying, stylus-driven shooter sequence, where you fly the casket up into space in order to purify the evil being inside. These segments tend to drag on way too long, but they're a fair bit of fun. They break up the standard overhead action RPG trappings of Lunar Knights rather nicely.
Lunar Knights is a worthwhile DS endeavor, especially if you're jonesing for something in the vein of Zelda that doesn't feel like a complete rip off. Even though the game's pacing and long-winded conversations don't always make for the most ideal portable gaming experience, this is still a really solid adventure.