The plot follows the fortunes of the Somtaaw, a small clan kept in stasis for most of the Homeworld game, thawed out late in the resettlement of the homeworld and with decimated populations, these smaller clans had been forced into space as the best of the land went to the clans of the Mothership's crew. On a mining expedition, the Somtaaw's base-ship finds a high-tech pod and takes it aboard to investigate it. Needless to say, the pod isn't entirely a benign entity and the story develops from there, masterfully and emotionally into probably the best RTS plot I've ever been through and maybe even one of my favourites ever.
Combat is more punchy than Homeworld and production is faster, too, leading to a much more resource-led war than was fond in the original game. While this leaves the game pretty much without the "must keep all my ships safe" preservation-orientated combat of the first game, it replaces it with a much livelier system of tactical strikes, feints and repair-retreats that the original lacked due to the fact that a tactical retreat sacrificed too many of your precious ships and resources to be worth the bother most of the time.
Following on from the storyline of the original, Cataclysm's gameplay is a definite improvement over even the high standards set by Homeworld, with an intuitive hotkey system supplementing the old, cumbersome menu system. Homeworld featured a bewildering array of ships which many found confusing and hard to keep track of. While Cataclysm has streamlined the ship selection somewhat, it feels like they've cut back a little bit too far, leaving the player's side feeling a little lightweight compared to the original.
This minor criticism aside, Cataclysm has great dramatic tension and gives you such an emotional connection to your forces that you will find yourself returning to this one quite a few times, even after you've completed it. Highly recommended.