Chandler examines his wound.
Slattery checks in with the sonar team. The sub is using a search pattern, and is closing in on the Nathan James. Instead of continuing to evade the Achilles, Chandler directs the crew to move the ship to a position for attack.
Valerie takes a shot at Granderson, who stands up to her. They work together to get the communications program up and running.
Chandler briefs Michener on the plan to attack the Achilles. He wants to offload all the civilians, including the President.
Civilian boaters are on the hunt for the Nathan James.
Valerie explains how she hacked into the Deadman system and is able to track the movements and plans of the Immune and the civilians alligned with them. The officers come up with a plan to use Deadman to lure civilians away from danger, and Ramsey to a place where they can attack him easily.
Burk is depressed, cynical, and despondent when another crew member mentions his home is nearby.
Miller and Wolf discuss the fallen crew members, Michener bonds with one of the kids, and Tex makes sure that Rachel is armed.
Foster and Green share a moment before they part on separate missions.
Slattery is going with the land team, and is disappointed that he'll miss the showdown with Ramsey. Chandler gives him a letter to miss to his family. The ship's civilian population and the President load onto the ribs with Slattery and the land crew.
The land team lands in the bayou to hide the president, Rachel, and the cure.
Jeter and Andrea work on the depth finder conversion.
Chandler and Foster plot the plan of attack over a map of the Mississippi River basin. Alisha and Valerie send out the Deadman messages.
Ramsey receives the false messages about the Nathan James location. He has crews on land as well to cover all contingencies. He sets course for the Nathan James supposed location.
The ship approaches where the point where the first blockade was before the messages were sent out. He orders that the ship should not shoot to kill at any remaining American ships.
The ship approaches the blockade checkpoint and sails through without incident.
Slatterys team findss a place to bunker down, and the kids plan to eat.
The Nathan James continues on, and Andrea, Jeter, and O'Connor do a test run on the depth finder.
Tex reassures Rachel that both they and the ship will be fine. Two travelers approach the campsite, one a sick child. Rachel wants to help, but Slattery tries to stop her. She goes out anyway, but their companion has a gun and warns her off. She removes the sidearm given to her Tex, and approaches the family. The father reluctantly accepts her assistance. Rachel approaches and hugs the child, sharing the contagious cure. The parents are confused.
The family was infected by the Immunes. The family was heading back to the home to die. Tex suggests that she inject the family since she has never tested the cure on humans. The family shares news that Ramsey has a weapon to sink the ship. Slattery takes Tex and Green to deliver the message to the ship, leaving Burk in charge.
The sub receives more sightings of the Nathan James. Ramsey realizes that the ship is moving to quickly, and that Val is helping the Navy.
The ship goes silent as it prepares to coast into position. The depth finder is fired up to find the sub, but it does not show up.
Ramsey's sonar operator thinks he heard something, but decides it was just an echo.
Chandler has Foster prepare a missile to fire, but he does not have a target. The captains wait each other out. Chandler adjusts course, and orders another ping from the depth finder cum sonar. Ramsey orders a prop turn. The ships see each other on sonar. The Nathan James is literally on top of the Achilles.
Chandler can't fire from his current position, and neither can Ramsey. A tense game of cat and mouse follows. Ramsey prepares to fire and dive. The Nathan James loses sonar capabilities just as the Achilles is able to dive into deep water. Working off of intution and years of knowledge, Chandler makes moves to avoid the torpedoes. There is an explosion in the water, but the crew doesn't know if it was the Achilles or decoys. The Nathan James is hit, and portions of the ship are sealed off. Jeter, O'Connor, and Andrea are locked in. The Nathan James is out of weapons.
The doctors document the girl's recovery.
Chandler heads to shallow water so that they can use ground weapons against the sub. Ramsey pursues.
Tex, Slattery, and Green find truck tracks. They follow them to find the Immunes with four warheads and launchers. They are able to see the Nathan James from their position. They launch a diversion to distract the men manning the weapon from shooting at the ship.
Chandler heads into shallow water despite a warning that the ship could bottom out.
An Immune attempts to fire at the James in the midst of the fight. The Achilles rises and rushes at the James.
Danny seems to be hurt and Slattery is in close contact with the man who was helming the warhead.
The sub is locked on the James, but the James doesn't have a shot. Suddenly a missile hits the sub. Slattery used the warhead to sink the Achilles. The sub crew attempts to abandon ship, but Ramsey won't allow it. Slattery fires again at the sub, and it sinks.
The call comes through to return to the ship.
The sub sinks to the bottom of the ocean, with Ramsey still alive and conscious.