The Players' Introduction, or back page text, tells of ancient swamp dwellers exacting revenge for those they feel have abandoned them. The DM's Background explains a bit more - how the relationship between swamp-dwellers and villagers arose, and why the swamp-dwellers had wanted the alliance in the first place (something they conveniently omitted to tell the villagers). It involved a dragon, of course. One that was crippled and resorted to other ways to exert its power over its surroundings.
The adventure starts in a nearby town, where the sole survivor of the village has sought aid. This has set the entire town on edge, noticeable as the party arrives. In due course, the party will be offered work, their mission to find out what is going on in the village and why they were attacked. It's up to you to get them there: some frontier terrain, trackless and marshy, with a couple of encounters thrown in, is recommended but not provided. The devastated village is well mapped and described, with an interesting twist - the invaders actually want to talk! Parties who rush in to do battle straight away will not do themselves any favours.
There's some excellent scenes where the true otherness of the intruders is revealed, and the party will have to engage with their customs to make the best impression before venturing into the swamp itself to confront the problem that brought matters to a head. There's some interesting follow-up material to suggest directions in which your plotline might go given the outcome of this adventure.
Overall, a neat low-level advanture to give a beginning party a chance to make its name, with good portrayal of other races as really being quite different, requiring cultural adaptation to deal with them - they are not just enemies there to be slaughtered: they have reasons for what they do.