When I say underrated this time, I actually mean underappreciated. Serenity is, in fact, highly rated by those who have seen it, but it seems that so very few people actually have. Based on the short-lived and cancelled TV show, Firefly, Serenity is a science fiction/western hybrid and the creation of Joss Whedon, the man who brought us Buffy and Angel on TV.
As I understand it, Firefly was completely shafted by Fox when it was aired in 2002 (possibly partly because of its anti-establishment undertones) - apparently Fox skipped episodes, played episodes in the wrong order and messed the show around with its time-slot - and was then cancelled by the network after only 11 episodes (14 were made in total). In that short space of time it garnered critical acclaim and, more importantly, a devoted fan-base. So that after it was cancelled, the passion of the fans, the writer and the cast kept the flame alive, so to speak... and in 2005 they made the movie Serenity. It is essentially an extension of the story of the TV show, but in motion-picture form (so it of course needs to be able to stand alone as a story). I had not seen the TV show before seeing the movie, but I'm sure that the experience was no less rewarding as a result.
With no major stars and no real promotional budget behind it, you could be forgiven for letting it slip under your radar. But I strongly encourage you to see it. It is well made, well acted, with a strong story and history, it has excellent and complex characters - and, as all sci-fi's should, it has special effects that are there to help tell the story rather than be the story themselves. Since seeing the movie I have bought the TV show on DVD and, so far, am enjoying it immensely. Here are some links to help you in what should now become you quest to watch Serenity and Firefly:
Serenity on DVD (UK)
Firefly on DVD (UK)
Serenity on DVD (US)
Firefly on DVD (US)