The game is a companion to the series, with a huge budget of 70 million dollars and that Trion promises to continue to support even Defiance, the series is canceled. For now we must evaluate both the series and the game at the same time and see how forcefully unite the two, because only then we can excuse some weaknesses of the game. If you do not see any episode of the series will feel lost, especially as regards the characters on which the game does not explain anything.
Regarding the game itself the best way to start would be to compare it to Borderlands. The gameplay is very similar, as well as all the general scenario. The story, which should really know the series, tells of a not too distant future where mankind goes to war with alien refugees of various races. The war Terraforma Earth, transforming it forever both in appearance and the creatures that inhabit it.