Marine architect Hannah Bryson was hired to inspect a decommissioned Soviet submarine, Silent Thunder, that was recently purchased by the U.S. government for use as a maritime museum. Before she and her brother can sort out a cryptic mssage discovered behind one of the ship's panels, there's a deadly assault on the sub, and Connor is killed. Though the government tries to warn Hannah away from the danger, she'll stop at nothing to find the ruthless mastermind behind her brother's death. Even if it means joining forces with a mysterious man who may be even more dangerous than the enemy she has sworn to bring down.