When two women are shot dead in the basement of a rented house, Nikki and Jack are baffled by the conflicting possible motives for the killings. Meanwhile, Leo is tasked with proving that his one of his pathology colleagues and close friends made a mistake when giving evidence that imprisoned a mother for smothering her baby to death. As the hunt for the basement killer intensifies, Nikki nearly meets her death when evasive D.I. Reed leaves her in a burning warehouse to pursue the chief suspect. Meanwhile, Clarissa and Jack are shocked to uncover a dangerous plot that threatens national security - including an outbreak of a deadly disease - while Leo must race against time to prove the imprisoned mother’s innocence, and reunite her with her other son, Darius, before he is lost in the adoption system.