When Special Agent Will Trent arrives in Grant County, he finds a police department determined to protect its own. Officer Lena Adams is hiding secrets from him, and while her role in the death of the county's popular police chief is unclear, that man's widow, Dr. Sara Linton, desperately needs Trent's help to crack the case of a prisoner's death. While the police force investigates the murder of a young woman pulled from a frigid lake, Trent investigates the police force. Caught between two complicated and determined women, trying to understand the facts surrounding Chief Tolliver's death, Trent will uncover explosive secrets - and confront a thin blue line that could be murderous if crossed.