The story, such as it is, basically is Frankenstein's monster retold. Humanity in an effort to make better war genetically creates the BioDerms as perfect genetic soldiers for use in their futuristic interplanetary tribal warfare. But these soldiers grow resentful and rebel against the humans, giving the rival tribes a new common enemy that also happens to be perfect killing machines. To be honest the BioDerms are a wash and a waste of time. They add an old cliché to an already threadbare storyline and the 'Derms, as they're called, look fairly stupid. Like something out of an ill-considered first-person shooter rather than ripped from more interesting sci-fi. They should have resurrected the evil Cybrids from Earthsiege and later Starsiege as bad guys, they're much cooler, more developed as enemies, more 'alien', and nostalgia value has longer legs than creatures that look like cheap space orcs.
One thing they did add was a lot more diverse environments to play in. Through necessity Tribes 1 took place on sparse tree-less planets. Tribes 2 places us in swamps, deserts, snowy gorges, and mountain range locations. The powerful engine still delivers seamless outdoor vistas (you can literally fly or run to anything you see on the map) yet doesn't balk at having to move into an indoor environment for a little corridor/base interior firefighting.