“You’re the one they want. The one they understand,” Elizabeth Jennings, or rather Nadezhda, a K.G.B. double-double agent under deep cover, tells her husband, Philip, in the season finale of “The Americans.” Elizabeth, played by Keri Russell, is explaining why she should take the more dangerous of two missions—the one they think is a setup—and Philip should be ready to put their two kids in a car and make a break for Ottawa if it all goes wrong. It does, in ways no one involved foresees; the episode, like the series itself, is about delusions—romantic, political, bureaucratic, tactical, marital, fashion (the year is 1981). And parental: Can Elizabeth really think that her children “understand” a father whom they believe is a travel agent but is actually a spy and assassin who’s just staged a sham wedding with a deluded F.B.I. secretary at which their mother pretended to be his sister? Can the K.G.B. really think that Al Haig might attempt a military coup after John Hinckley shoots Ronald Reagan—a major plot element in an early episode? Maybe they can.

It’s often said, admiringly, that “The Americans” is a show about marriage that is dressed up as a spy drama. One of its premises is that marriage itself is a matter of dressing up and performing, and that those enactments, particularly when children are watching, can be its most genuine part. Paige, the Jennings’s thirteen-year-old daughter, and Henry, her younger brother, watch their parents like spies. They are the hard pegs in a marriage constructed by the K.G.B. as cover for their parents, whose decision, early in the season, to figure out whether they have fallen in love with each other leads to problems on the job and a separation. (“Hitting the pause button,” as Philip describes it, when they give the children the news over a basket of fried chicken.) It’s familiarly sad.

But if the show were just about marriage it would only be half as interesting. There are many shows about men and women and children; fewer—maybe just one—in which so many characters are obsessed with the Strategic Defense Initiative, the space-based anti-missile Star Wars dream. I’ll have some spoilers here, including that the great revelation of the finale—the secret that Philip and Elizabeth think they’re risking everything to get, from an Air Force colonel who’s set up a meeting on a park bench—is that S.D.I. doesn’t actually work. The technical plans are “a fantasy.” (As, indeed, they were.) The only thing that may be real is Reagan’s enthusiasm. Nor is it clear that the truth matters to anyone; this isn’t “War Games” for grownups.

The American farce does not serve to redeem the Russians, either. Philip and Elizabeth are less antiheroes than studies in what leads apparently earnest people to behave cruelly. They represent, respectively, the banality and the ideology sides of that debate; Philip, who doesn’t seem to care much about Marxism, is at times the more ruthless than Elizabeth, who does, as when he grabs a pillow to smother the son of Caspar Weinberger’s housekeeper. They both kill a lot of people.

The real antihero may be Stan Beeman, an F.B.I. counterintelligence agent who is also the Jennings’s neighbor, played by Noah Emmerich. (Tad Friend writes about Emmerich in the magazine this week.) Stan started the season as a amiable threat, a G-man version of Jack McGee from “The Incredible Hulk.” (Matthew Rhys, who plays Philip, has some of the affect of Bill Bixby.) Now he’s carried out a revenge killing, sleeping with Nina, a young woman in the K.G.B.’s Washington rezidentura whom he’d blackmailed into sleeping with and then setting up her old boss, and generally messing up his marriage. Nina is now a triple agent, having confessed to Arkady, the new head of the K.G.B. office. (Arkady is one of many strong characters, along with Martha, the tragic F.B.I. secretary, and Claudia, the Jennings’ handler, who plays Pac-Man and gets lines like, “The first time I saw him he was standing over two dead Nazis.”)

Nina re-pledges her loyalty to the Party and State out of some mixture of personal, ideological, and maybe even patriotic motives—and the way that each of those factors has some reality and irrationality to it is, again, the show’s strength. (The spy scenes also work well in terms of craft, maybe because the head writer, Joe Weisberg, spent time with the C.I.A.) “The kind of man who did what was done to you, Nina, is weaker and more vulnerable than he seems,” Arkady tells her. He’s talking about Stan but could be describing himself or, for that matter, the Soviet Union, or any country that has lost its bearings.

If the show has a weakness it is, oddly enough, an unwillingness to fully commit to the early eighties, Henry’s “Star Wars” bedspread notwithstanding. It has more of a second-term Reagan feel, leached of the sourness of the seventies, with an admixture of Eisenhower. There is a school assembly with a heroic astronaut. Part of this is production design: only token graffiti and garbage, an unwillingness to give Keri Russell puffy hair. (Ironic, I know.) More than that, the one sort of complexity the show seems to have trouble with is the American kind.

In a single serpentine plot line a few episodes back, the show killed off two of its best characters and, not incidentally, its only two visible non-white ones: Chris Amador, Beeman’s partner; and Gregory Thomas, a black man working with the K.G.B., played by Derek Luke. This felt like a structural mistake, particularly in the case of Gregory. In the marriage plot, he created a triangle; there is an awful lot of transactional sex in “The Americans,” but his relationship with Elizabeth is portrayed as going beyond that. So do his reasons for working with the Russians. Elizabeth recruited him at a meeting of Martin Luther King’s Southern Christian Leadership Conference. He wanted to make a difference. He runs surveillance teams and gets VIN numbers filed off cars. He talks about the K.G.B as if the term had glamour.