Silent Service

Harm and Mac find themselves too close for comfort when they are sent to investigate an international incident on a nuclear submarine.
When Harm and Mac, who are already getting on each other's nerves, find themselves in the cramped quarters of the submarine, their relationship is stretched to the absolute limit. Their attempts to stay out of each other's way as much as possible are futile since they must work together to find out why the sub surfaced miles off course, sinking a Norwegian boat in the middle of a regatta.
Initially, it seems that the incident was the result of reduced staffing due to a breakout of E-coli. However, as more health problems begin to arise aboard the sub, Harm and Mac suspect a more criminal element than disease.

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S04E17 - Nobody's Child