From the developer's website:
An aquatic alien race secretly launches a devastating attack on Earth which melts the polar ice caps. Water covers the planet.
Humanity is all but wiped out by this onslaught, but some of the remaining humans band together to take the planet back from the brink of chaos. Using existing technology, the Iron Marines take to the seas as they struggle to survive in this wet new world.
For over one hundred years, Captain Nemo and his followers have been living peacefully and quietly at the bottom of the ocean, sharing an uneasy truce with the highest officials in the British and American governments. In their secret utopia the Nemoidians have isolated themselves from the world and lived in peace – until now. Nemo is enraged at the disturbance to his oceans.
As the ice caps melted, the ancient city of Atlantis began to thaw. Trapped by the last ice age, this mighty civilization disappeared at the height of its power. Now free, the Atlantians will stop at nothing to reclaim the planet they once owned.
Having escaped from their cruel overlords, the fanatic Elect put their hope in the Ark – a massive city-sized space craft that could deliver their dying race to a new world. The fuel supply of the Ark had been nearly depleted when the aliens finally discovered Earth, a planet perfect for transformation into a new, oceanic home for the aliens...
Undertow features a 15 level campaign for single-player or co-op and two robust multiplayer modes for up to 16 players. While many of the maps have level-specific goals, the core gameplay of Undertow centers on this maxim: Kill as many enemies as you can – and stay alive for as long as possible!
Players select their race (Iron Marine, Nemoidian, or Atlantian) and unit type and are then launched into battle. Each of the three races features four unique, upgradeable unit types ranging in ability from the fast and agile to the large and powerful. Players have the ability to unlock and purchase unit upgrades as they acquire points. Each unit type has three levels of permanent upgrades.
Through conquest-style battles, players obtain points by killing enemies, accomplishing team goals, and taking check points. Points and a variety of power-ups can be used to further upgrade unit abilities.