Plague of Shadows is an add on that uses all the same levels from the original Shovel Knight, while adding some new areas within those levels. Since it is pretty much the same game, it is Plague Knight's gameplay mechanics and his story that makes it stand apart from the original.
I think you will get the most enjoyment out of the story if you have already finished the original Shovel Knight. It is not a requirement, but there a lot of references, both obvious and not so obvious, that add a lot to the story.
As for gameplay, Plague Knight has a whole different set of weapons, special abilities and movement mechanics from Shovel Knight. His main weapon, a bomb, is highly customizable, allowing for different types of gameplay. I found myself focusing on a few combinations that helped me get through the game.
With the change of movement mechanics, the reuse of the old levels from the original Shovel Knight works. It feels like a new game. I also found that the levels were more difficult with Plague Knight than with Shovel Knight.
My main problem with Plague of Shadows is the same as the original Shovel Knight. The bosses are just too easy. Plague Knight makes the majority of bosses much easier than they were in Shovel Knight.
Overall I enjoyed Plague of Shadows as much as the original Shovel Knight, if not more. The story and platforming are so good that they outweigh the easy boss battles.
I recorded a video of the intro, first level and town. This video is from Hard Mode, where you carry over all unlocked abilities. You can view it here:
My original Shovel Knight review is here:
Yacht Club Games provided Cheerful Ghost with a download key for Shovel Knight: Treasure Trove for the Nintendo Switch. Shovel Knight: Treasure Trove is also available for the Wii U, 3DS, PS4, Vita, PS3, Xbox One, Mac, Windows, Linux and Fire TV.