Earthworm Jim 3D is the characters first foray into the third dimension. As Earthworm Jim is sent into a coma by a flying a cow, he must somehow infiltrate his own mind to set things right...? I don't know how that makes sense.
Regardless, you're Earthworm Jim, you're in your head, you've gone nuts, and you've got to fix it. Using 15 weapons and basic abilities like jumping, shooting, and whipping your head around to defeat whatever enemies lurk in your brain. The point is to gather all your marbles (You see, you lost your marbles) and golden udders. The udders part doesn't make sense either, but that's how the game is - it's wacky and unpredictable.
If you're wondering how the 2D to 3D transition went, by all accounts, it plays just like the 2D classics in 3D. Except for some camera problems and, like most 3D platformers, significant fetch quests and item collection.