The Cove has been getting great reviews: four stars from Ebert (who calls it "a certain Oscar nominee") and a score of 82 on Metacritic. A quick synopsis from Wikipedia followed by the trailer:
The Cove is a 2009 documentary film documenting the annual killing of more than 2,500 dolphins in a cove at Taiji, Wakayama in Japan. The film was directed by former National Geographic photographer Louis Psihoyos, and was filmed secretly during 2007 using underwater microphones and high-definition cameras disguised as rocks.