While I find your gameFAQs quick review (TM) of Monster Hunter Tri quite insightful, I am still concerned about the exact complexities of the game's plot. I would rather not purchase a new title only to have major overarching themes fly over my head. Is it possible to absorb such a grandiose story while lacking knowledge of the series' history?
The Monster Hunter series does not follow a linear plot. However, themes are shared between Tri and it's predecessors. Familiarity developed with the "hunter" archetype in these past titles will definitely have an influence on the emotional impact of Tri's story and on your enjoyment. On the other hand, coming straight into Tri without any previous experience in Monster Hunter titles will keep you from seeing the truth of the Monster Hunter world prematurely and add impact to the plot developments.