Fast-paced underwater shooting is at its best with friends.
Story is told through cutscenes and the events in the game move forward very rapidly, which is probably a good thing in an Xbox Live Arcade game. There's no real depth in different characters, but in the future there are some established underwater utopias which are in war against one another. This mish mash also includes Atlantis and some aliens, but it's over before you really get the hang of things. So in a word the story in Undertow is weird and pretty much meaningless.
Graphics are surprisingly good especially in cutscenes, but the actual action is also beautiful to look at. Frame rate stays solid at all times. Sound deserves credit especially in its voice acting, which is done well all the way. Firing a gun underwater really sounds like it so in my opinion the audio is the best part in Undertow.
Gameplay is simple 2D-shooting, where you aim by using the right thumbstick and the left thumbstick is used for navigation. The goal is to shoot enemies and take control of control points from the enemy and at the same time keep your own control points to yourself. It's all very simple and it stays that way always, no matter what happens in the actual story.
Game consists of 4 Acts which is to say that it's very short. On the other hand you can't really expect a Live Arcade game to be very long. Saving is done between missions and this aspect works fine.
There are 3 difficulty settings to choose from, but there are major differences between them: Casual was way too easy, but Normal was maybe too much. I recommend playing on Casual the first play-through and on Normal with friends (up to 7 of your friends and you can participate).
Undertow is a fun snack from Xbox Live Arcade and it's at its best with friends.