The extended 85-minute episode sees the culmination of Abu Nazir's terrorist plot at the Vice President's summit, while Carrie Mathison's downward spiral continues.
That night, Tom Walker (Chris Chalk) commandeers an apartment overlooking the State Department, which is the site of the Vice President Walden's upcoming policy summit.
As Walker continues to fire into the crowd, the surviving VIPs are rushed into a secure bunker beneath the Harry S Truman Building.
During the conversation, Carrie realizes that Brody has been sequestered with Walden, and tries to tell Saul that the elimination of the Vice President and the other high-value targets is the true goal of the attack.
Inside the bunker, Brody approaches Walden and attempts to detonate his explosive vest, only to find that the device has become disabled owing to disconnected wires.
Meanwhile, Carrie arrives at Brody's house and encounters Dana, urging her to call her father and talk him out of the attack.
He demands to know the story behind the covered-up drone strike, and has brought along a big bargaining chip in the form of evidence that Walden authorized torture when he was head of the CIA.
This conversation is Carrie's breaking point: With her life and career in a shambles, she now even doubts her own sanity, as nothing came out of her theory about Brody (she has no idea she actually stopped his attack).
Brody explains to Nazir that his vest malfunctioned, but that maybe it is a good thing that it did, as he is now a trusted ally of the man who is going to be the next President, and will be able to influence him.
Saul barges into Carrie's hospital room, where she is being prepared for electroconvulsive therapy (ECT) in hopes of treating her bipolar disorder.
[1] Michael Hogan of The Huffington Post called "Marine One" "a monumentally satisfying, forward-leaning episode, one that resolved many of our most burning questions while leaving plenty of room for future drama".
HitFix's Alan Sepinwall said this should surely be Damian Lewis' Emmy submission episode and that his acting was "raw, magnetic, unflinching, mesmerizing".
[7] Maureen Ryan of The Huffington Post thought the two leads delivered "breathtaking performances" and said "I don't think I've ever seen anyone portray vulnerability and pain as well as Claire Danes; she projected Carrie's deep depression like a force field of weighty despair".
[8] Strikes against the episode were generally considered to be a lack of resolution, and some major contrivances in the climactic bunker scene.
Matt Roush of TV Guide praised the episode but said the resolution to the bunker scene was "awfully convenient, not as satisfying a twist as we're used to from this taut thriller", but that "it does reflect Homeland's unusually emotional context".