The British are coming! The British are coming! To Manh(a)ttan, that is. This week’s episode (“The Second Coming”) finds our intrepid community of physicists eagerly awaiting their own British invasion: the arrival of an elite contingent of some of the greatest scientific minds in Britain, come to join forces with the Americans (and European refugees) in the race to beat Germany to the first atomic bomb.

The excitement in the Winter lab is palpable, as Frank’s scientists try to guess the identities behind the pseudonyms given to the British physicists, which tend towards the erudite and literary, if you’re being generous, or pretentious, if you’re a crass American (e.g, “Stephen Blackpool,” a character from Charles Dickens’ Hard Times). There’s a catch: the new arrivals will get to choose whether to work on Ackley’s Thin Man design, or with Winter’s implosion group. Fortunately, Winter has his own resident Brit in Paul Crosley, who offers to serve as a cultural liaison: “There is an art to properly seducing an Englishman.”

Charlie Isaacs is oblivious to the coming invasion. Since getting back from Oak Ridge and having his troubling insight into a potential death knell for the Thin Man bomb design, he has been working feverishly through the nights, going over every calculation concerning the properties of plutonium isotopes. The issue, you may recall from last week, is that Pu-239 made in a reactor quickly becomes contaminated (thanks to a pesky xenon isotope) absorbs extra neutrons flying about the reactor [thanks to a commenter for pointing out the error] and turns into Pu-240 — which undergoes spontaneous fission and thus would set off any bomb prematurely, before it could be fully assembled for deployment.

Charlie rushes off to Helen’s room in the middle of the night to tell her this, and is soundly rebuffed when he asks to come in so they could figure out what to do with this insight. It’s mostly because Helen is not alone: her new paramour, Paul Crosley, is there. And, you know, good for Helen. But it leaves Charlie without his former ally, caught in another crisis of conscience.

Charlie Isaacs (Ashley Zukerman) ponders whether to reveal his discovery of a critical flaw in the Thin Man bomb design on "Manh(a)ttan.

There is one other person Charlie can trust: Theodore Sinclair, the black nuclear physicist he met at Oak Ridge, whose specialty is the byproducts created in nuclear reactors. He makes a deal with his wife, Abby, still working as a switchboard operator: if she arranges a non-monitored phone call for him, he’ll look the other way when she leaves the base without permission to meet her parents, who are passing through by train en route to California. Abby does so, except she breaks her promise to Charlie and listens in on the conversation with Sinclair.

The verdict, when it arrives, is not good. Later that day, Charlie gets a message from “Christopher Columbus” (Sinclair’s chosen code name): “400,000.” What does it mean? It’s a reference to the spontaneous fission rate of Pu-240. Physicists certainly knew at the time that even-numbered isotopes undergo spontaneous emission, but everyone assumed that this wouldn’t be a problem as long as the rate was “under 300.” So Sinclair’s calculation that the number is 400,000 for Pu-240 is truly disastrous.

And here we get an interesting artistic choice by the writers on how to convey Charlie’s inner turmoil about what to do: a waking dream sequence, whereby he marches into Ackley’s office and tells his boss the Thin Man design won’t work with reactor-bred plutonium, and Dream Ackley asks him why he told him, given his moral misgivings about building a bomb in the first place. Dream Charlie protests he was just doing his job. Dream Ackley tells him “You sounded the alarm. Now we’ll reorganize all our resources.” The bomb will be built, it will be detonated, and “all those lives will be on your head, Charlie.” Then Dream Ackley calls him a “conniving ungrateful little Jew,” which is when we realize this isn’t really happening. But it’s enough to give Charlie pause and not go immediately to Ackley with the news.

Further complicating the situation is Ackley’s decision to name Charlie as his second in command, in charge of the lab during Ackley’s absences, which are going to be more frequent in the coming weeks as they move out of the design phase to actually build Thin Man. He just wants Charlie to work a bit on his people skills, since he’s managed to alienate the rest of the team. Now would be a very good time for Charlie to spill what he’s learned about the design flaws; instead he waffles and asks to think about it, fearing he might not be good management material. Ackley is annoyed, but gives him a day.

Meanwhile, Babbit runs into Liza on base and is shocked to learn that not only is she barred from scientific research during her husband’s tenure on the Manhattan Project, but she can’t even work a menial job, like laminating badges, because she failed her polygraph test designed to ferret out any security risks. In fact, she can’t walk around the research area at all unless accompanied by an MP. She asks him to just forget about it, but Babbit is a loyal friend, particularly to Liza, who never had a problem with his sexual orientation and always respected his privacy.

He confronts the man who administers all the tests, interprets the results and reports them to security, and asks him to review Liza’s case, convinced it’s all just a misunderstanding due to a false positive. (This is a common occurrence with the very unreliable polygraph test.) When the man is initially dismissive, Babbit resorts to thinly veiled blackmail: review Liza’s case or he might just happen to mention that he shares Babbit’s own private secret — you know, a couple of middle aged bachelors who prefer the company of men.

The polygraph expert reveals it wasn’t a false a positive: Liza failed Question 17: “Have you ever received treatment or been institutionalized for a disorder of the mind?” We aren’t told what Liza’s particular disorder might be, but Babbit’s threat works: Liza gets her security badge allowing her to have some sort of job on base for the duration of their stay. He tells he the polygraph was “probably on the fritz that day,” so her clearance was denied in error, in an attempt to respect her privacy. Liza isn’t buying it: “Did they show you my file?” Babbit shrugs: “It’s none of my business.” So now Liza can have a job.

The British physicists finally arrive, and Ackley wastes no time laying on the charm, as Frank Winter and Paul Crosley assess the new recruits from a distance. Crosley is not impressed: “It looks like the A team stayed home,” he tells Frank — with the exception of one William Hogarth (Bill Camp). Frank recognizes the name immediately; Hogarth headed the British implosion project, and thus Frank is eager to have him on board, oblivious to Crosley’s sudden frosty silence when he recognizes Hogarth.

Paul Crosley (Henry Lloyd, right) Is Not Impressed -- with the British contingent on "Manh(a)ttan."

Whatever Crosley’s reasons, he sucks it up like a good soldier and joins Frank at a dinner with Hogarth, who reveals himself to be a drunken boor of the first order, smacking their server on the derriere and propositioning her crudely, while downing oceans of very expensive wine with the directive to put it on “Lord Crosley’s” tab.

Yes, in addition to be being a brilliant physicist, Crosley is English nobility, a few notches down in the succession for the British throne. And Hogarth bears some sort of grudge towards him underneath that forced joviality, perhaps because the highly eligible Crosley wasn’t interested in marrying his daughter. He’s also a recent widower, and when Frank tries to broach implosion, he makes his terms for joining the Winter lab crystal clear: he wants sex, i.e., access to the entrepreneurial prostitutes on base.

And that’s how Frank and his entire team end up awkwardly listening to the sounds of Hogarth’s enthusiastic rutting with a string of young women for hire, since “Lord Crosley will happily pay.” But then he spies Jeannie — the young woman Fritz has been “dating” — and Crosley finally snaps. He intervenes and pays Jeannie not to have sex with Hogarth. “A slave would do well not to poke the lion,” Hogarth sneers,” adding, “She named him Henry, by the way.”

Yikes. Apparently Crosley did take a shine to Hogarth’s daughter, but abandoned her when she became pregnant, leaving England to join the Manhattan Project. We knew he was a player; now he’s revealed as a cad, a bounder, a deadbeat dad. Frank intervenes as the tension escalates, and Hogarth declares that none of the Brits will be joining Winter’s “group of misfits and liars.” As for Frank’s implosion design? Hogarth declares it a pipe dream, claiming he had disproved the theory and would send over his research so Frank could wallow in his pathetic failure. “Get this Limey prick out of here,” Frank snaps. Oh well. At least Jeannie didn’t have to endure intimate relations with the revolting Hogarth.

The key issue, alluded to in earlier episodes, is controlling the shock wave from the implosion — something Hogarth declares is “out of reach.” Historically, a Los Alamos physicist named Seth Neddermeyer developed an implosion model for the design, in which explosives were used to compress a plutonium sphere (the “pit”) so rapidly, its density would induce criticality and set off a nuclear chain reaction. (NB: Frank Winter’s character might have been partially inspired by Neddermeyer, but Winter is still 100% fictional.) But the shock wave from the implosion didn’t last very long before dissipating outward, and if it happened too quickly, only part of the pit would be compressed at any given moment as the shock wave passed through it, halting the fission. What was needed was a way to hold it all together into a dense critical mass for a mere few nanoseconds longer so the fission wouldn’t sputter out (we’ll have more on that in future recaps, but feel free to discuss at length in the comments).

Abby Isaacs (Rachel Brosnahan) surprises her parents with a visit.

Abby Isaacs has been assiduously avoiding her fellow telephone operator, Elodie, since their drunken tryst last week, even switching her shift at work. Elodie sends a gift: a pretty bra, whether hers or Abby’s, it’s unclear. But the intent is unmistakeable.

Abby has other priorities, anyway: surprising her parents on the train from Santa Fe to Albuquerque, her little boy in tow. It goes about as well as most family reunions: initial hugs, a bit of polite conversation, and then her mother bursts into tears. The family has relatives in Minsk, on her mother’s side, cousins Abby has never met, who weren’t able to escape the country when the Nazis invaded.

Her parents have learned that the Jewish ghetto in Minsk has been “liquidated.” Those cousins are most likely dead, along with their toddler. They ask if Charlie can help find out for sure, but Abby says (correctly) that he can’t; his job is the science.

The Minsk ghetto was one of the largest in Eastern Europe, housing around 100,000 Jews, almost all of whom died during the war, starting with the intelligentsia, although large swathes of the population were routinely murdered from its establishment in 1941 until the final “liquidation” on October 21, 1943. By then, only about 9000 remained, most of whom died in the liquidation.

The sheltered Abby doesn’t want to hear about any of this at first, but the brewing quarrel with her parents is cut short by the arrival of a government official, who has tracked Abby to the train. She only had a day pass to leave base for Santa Fe, not hop on a train to Albuquerque. She ends up in Colonel Cox’s office, and slyly bursts into tears, explaining she is distraught about wanting to know the fate of her relatives in Minsk. He doesn’t give her any news, but lets her off the hook with a warning.

But Private Dunlavy, who is holding the baby while she meets with Cox, overhears, and shows up later that night with all the classified information he could find about what happened in Minsk. And the blinders fall from Abby’s eyes as she sees definitive proof of the horrific genocide taking place in Europe. Charlie comes home to find his wife tearful and shaking, not just over the death of the cousins she never met (and now never will) but over the sheer magnitude of the numbers: over one million Jewish people executed (six million by the time the war ended). “I don’t even know how to think of a number like that,” Abby admits. And while there is nothing she can do about it, Charlie can — by helping the Manhattan Project succeed in its mission to build a bomb.

That turns out to the catalyst spurring Charlie to action, but he still doesn’t go running to Ackley: he knocks on Frank’s door instead and tells him that Thin Man is “dead in the water.” Frank has been poring over Hogarth’s papers in dismay and tells Charlie that implosion seems to be dead in the water too. Without a veritable army of scientists, there’s no way he can solve the shock wave problem. “So…. what are we going to do about it?” Charlie asks. And Frank lets him into the lab. At last these two adversaries are working together, as they should have been all along. With four more episodes to go in this season, I expect we’ll see some significant developments on both scientific and professional fronts in the next few weeks.