Even as Marcus and Sam learn that they have been charged with treason against the United States, a greater threat within their own crew is brewing. Meanwhile, an old friend of Sam's, Paul Wells, pays Christine a visit and tries to ease her fears about her husband.

The crew is hit with an attack by an unknown force unlike anything they've seen before. As confusion and aggression take hold, Marcus and Sam can no longer trust their own senses even as they race to save the sub and defend the island.