Lake of Betrayal explores the history of Kinzua Dam on the Allegheny River in Pennsylvania and its impact on the Seneca Nation of Indians. Completed in 1965, it was originally proposed to help mitigate flooding in Pittsburgh, almost 200 miles downriver, but the 27-mile reservoir that formed behind it inundated vast tracts of the Seneca Indians' ancestral lands, forcing their removal in breach of the United States' oldest treaty then in effect. The film looks at the Seneca Nation's fight to protect its sovereignty against the U.S. government's Indian termination policy and overwhelming political and economic forces driving the post-WWII boom.