Marcus and Sam engage in a mock debate. They are prepping for a face-to-face negotiation with William Curry. The Secretary of Defense motors up to the island aboard one of three Zodiac boats. A dozen heavily-armed MPs serve as an escort. Curry is joined by White House advisor Amanda Straugh. Admiral Shepard is also part of the group. Marcus believes the government sent him because he's an old friend. As for Shepard, he likely wants to see his daughter. Unfortunately, Grace is busy at the moment.
Sophie reveals that there's a dead battery in a spot along the sonar array line, which is what keeps submarines from sneaking up on the island. A repair needs to be made 300 feet below the surface of the water. Grace is in command of the Colorado as it heads out to make the fix. She's working with a skeleton crew and the best diver they have available. That would be James. The mission is even more challenging thanks to Master Chief Prosser, who still refuses to give Grace any respect whatsoever.
Not a single member of the Colorado crew takes the Defense Secretary's offer to step away from their captain's command. Marcus loses his cool when Curry mentions his son's name. Guess that earlier mock debate didn't quite take. The negations ultimately continue even though White House advisor Straugh realizes the Illinois is close to finding the gap in the sonar array line. James must use a drill to change the battery down there. The Illinois will be able to find them the moment he flips the switch.
Marcus wants a public trial. He's willing to discuss the six Navy SEALS he picked up off the coast of Pakistan only after these terms are accepted. Curry delays his response. He's stalling until the Illinois gets a lock on the Colorado. Grace must dive the sub to avoid them. That means leaving her diver behind. James cuts his tether as his ride heads down to dangerously low depths. He continues with his mission to change the battery even though he'll likely not be able to survive once the job is done.
Admiral Shepard lets Marcus know that he's assembling a group at home that can take over the current U.S. government and expose the lies. The negotiations pick up again. Marcus claims he has evidence of the whole sordid mess of deception from his Navy SEAL pals. This newsflash has Curry making a very reasonable offer that is promptly declined. A better offer is made, but this time it's for Sam. All he has to do is turn over Marcus. They have a deal. As it turns out, it's one Marcus wanted all along. He's going down, but his crew will be saved.
The Colorado goes silent as it is surrounded. Grace knows that if they move they're dead. She gives the order to ready the torpedo tubes. She's hoping the other subs will back off once they show them their teeth. Grace makes a stirring speech to let the crew know that's she's betting on them as they head into harm's way. The Illinois acquires a firing solution on the Colorado. This means the offer for Sam is off the table. Curry tells Straugh to give the order to destroy the sub. Then...
BANG! BANG! BANG! Admiral Shepard shoots Straugh dead before the order to blow up the Colorado can be authenticated. Curry is hit in the leg before the MPs finally subdue Shepard. On the Colorado, Grace orders torpedoes fired into an underground volcano. On land, Curry completes the order to fire on the Colorado. KABOOM! Torpedoes explode on the volcano. The Colorado is able to get lost in the debris and avoid incoming torpedoes. Grace lets the outside world they have four nukes currently aimed at DC. She orders all forces away from the 200-mile perimeter and thanks them for their cooperation.
The Colorado has survived the day, but the crew fears James has not. The clock on his air supply has run out. The search is on, but there is no sign of life. Prosser tells Grace a story in which the message is to never underestimate a Navy SEAL. DINK, DINK, DINK. The tapping sounds heard outside the sub is James sending a Morse code message. He needs a ride home. Grace is happy to oblige. She's also moved by the salutes she receives from the crew. It's a simple gesture of respect that's led by the COB.
Paul Wells shows Christine Kendal a series of emails detailing how Sam and Marcus were planning to sell the Colorado and defect to China. It's a tough pill to swallow. Of course, the delicious meal Paul makes for Christine goes down very easy. These two "appear" to be bonding big time. Christine later has a "genuine" bonding chat with Kylie Sinclair, who has become persona non grata around DC. Christine now knows Paul Wells is a spy because there's only one thing Sam loves more than her. That would be his country.
On the island of Sainte Marina, Marcus lets Sam know that everything that went down there today never happened. At least, that's what the U.S. government will say if asked. The truth, however, must still come out to Grace as Marcus informs her about what happened with her father. As for James, he returns to share a kiss with Tani while Sam shares some coffee with Sophie. As for Marcus, he stands alone in his quarters gazing down at photos of the family he once had.