MDK2 HD takes place soon after the alien invasion that was the focus of the first game. Well, as it turns out, not all of the invading creatures actually left the Earth when Kurt, our heroic janitor, defeated the invading force. Realizing there was still some cleanup left to be done, Dr. Fluke Hawkins sends Kurt back out in his special suit to finish the job.
What results is a slightly different game than the first. Where MDK had you playing through the entire game as Kurt, the sequel alternates between the three heroes, Kurt, Hawkins and the robotic six-legged, gun-toting dog, Max. While you start off as Kurt and his gameplay feels much like the first game's style, it isn't long before the janitor is captured and Max is sent in to free him. Eventually though, Max is also captured and the Doctor must head into battle himself. What results is an odd series of rescue attempts that lead to only one character being free at a time, and you keep rotating between the different characters and gameplay styles.
Kurt has a chain gun built into his suit, and his Ribbon parachute system allows for some interesting gliding-jumps, but his main speciality is his sniper rifle. This gun allows him to not only deliver powerful shots at enemies, but also hit very precise targets, and often times, in the middle of a heated battle.
Max is all about running-and-gunning. You can equip up to four guns at a time, and when each gun's ammo depletes, you just exchange it for one of the others you've picked up along the way. Where Kurt tends to deal with long-ranged fighting, Max's abilities lend his fighting style to more close-ranged shooting.
Meanwhile, the game takes on a less action-heavy setting when Hawkins is in control. MDK2 HD feels a lot closer to an adventure title as the mad scientist has to navigate his way through odd obstacles and puzzles. While he doesn't face many enemies, he does run into a few, and he does develop a means of defending himself, but it's in a way totally atypical to the other two characters.
Since the HD part of MDK2 HD deals mostly with the graphics upgrade the game received, you shouldn't expect much of a change in the way of gameplay, so if you liked what you played back on the DreamCast or PS2, then you should enjoy what this version has to offer.