Over the years, the lavish German noir detective series “Babylon Berlin” has required patience from its American viewers.

First, fans had to untangle dense story lines — set across the late 1920s and early 1930s — along with an abundance of compulsively watchable characters, most of them harboring secrets. Then they had to wait years for new episodes. Netflix initially carried the first three seasons, but they were removed from the platform this year. Season 4 aired in Germany and elsewhere in Europe in 2022, but these newer episodes have only now become available to stream in the United States, arriving on MHz Choice, a platform specializing in European titles, on Tuesday.

This fourth season is “more music, more crime, more sex, more politics than ever before,” said Henk Handloegten, who created “Babylon Berlin” along with Achim von Borries and Tom Tykwer. It also features more Nazis.

Season 4 opens on New Year’s Eve 1930, and German politics is kicking into a higher, scarier gear. “Suddenly, our heroes are confronted with a completely new and profoundly disturbing and menacing energy: the rise of fascism and the far-right,” Tykwer said in a joint video interview with his two colleagues.

Based on a series of books by Volker Kutscher, “Babylon Berlin” sets fictional characters against a backdrop of real events, so we know that Hitler’s rise to power will end the democratic Weimar Republic. The season’s suspense lies in discovering “how these characters we’re getting to know are reacting to the upcoming Nazi period,” von Borries said. “Will they be on the right side or the wrong side?”

Here’s a little refresher of where we left things at the end of Season 3, and what to look out for in the new episodes.

What will happen to Gereon?

A key “Babylon” character is Inspector Gereon Rath (Volker Bruch), a World War I veteran who becomes a dogged inspector in the Berlin police department’s homicide division. Gereon is tireless and dedicated, and fundamentally believes in democratic values. He has been fair-minded toward communist activists, befriending the muckraking journalist Samuel Katelbach (Karl Markovics).

By the end of Season 2, Gereon appeared to have overcome his post-traumatic stress disorder by making peace with his brother’s death in the trenches, thanks to the mysterious Dr. Schmidt (Jens Harzer) — who may or may not in fact be Gereon’s brother, and who embodies the period’s curiosity for new approaches to mental health treatment, like hypnosis and psychoanalysis. Season 3 ended with Schmidt’s announcing: “We are creating the man-machine. An android free of pain and fear.” Uh-oh.

Inspector Ritter, on the case

The other main character is Charlotte Ritter (Liv Lisa Fries), a typist who has become the first female homicide assistant detective in Berlin. She and Gereon have a strong, mutually respectful professional relationship. Of course a question hangs between the pair: Will they ever be more than colleagues?

Lotte has a deep knowledge of the city’s nightlife, and she loves dancing — she goes out with gal pals or even Gereon, and used to make extra money to support her impoverished family by doing sex work at the glitzy club Moka Efti.

In Season 3 we started to see more of her sister Toni (Irene Böhm). The two had a bitter fight and Toni took off, but the siblings’s relationship, and the character of Toni herself, will continue to play out in Season 4. In fact, that sisterly bond puts Lotte in a deep jam at the very beginning of the new season.

Law and disorder

Since both Gereon and Lotte are detectives and each season features a new case, we also get to know their colleagues at the police station. The most sympathetic and supportive is Ernst Gennat (Udo Samel), who leads homicide investigations.

The most deviously dangerous is the slick Gottfried Wendt (Benno Fürmann), a committed Nazi who oversees political groups and activists — look out for the guy with the scar on his cheek. Wendt orchestrated the death of his Jewish predecessor and got away with it (though Gereon knows the truth), and he has grown into one of the show’s main antagonists.

In Season 3, the sympathetic police photographer Gräf (Christian Friedel, recently seen as the camp commander in “The Zone of Interest,”) was revealed to be gay and started a relationship with a journalist, Fred Jacoby (Peter Jordan); as we head into the 1930s, Weimar’s relatively relaxed attitudes toward sexuality will begin to change.

Money, money, money

In “Babylon,” the German establishment is represented by the industrialist Alfred Nyssen (Lars Eidinger, recently seen in “All the Light We Cannot See” and “Irma Vep”), whose considerable fortune grew even larger after he successfully speculated on the stock-market crash of 1929.

Alfred has a tense relationship with his mother, Annemarie (Marie-Anne Fliegel), who thinks he’s an idiot. In Season 3, he started getting close with Gereon’s former lover, Helga (Hannah Herzsprung), who is dazzled by his wealth and unconcerned by his ties with extremist factions — Alfred has long supported Germany’s secret rearmament.

The rise of Nazism

In the first two seasons, the anti-democratic forces at work in Berlin were mostly high-level military men who dreamed of restoring the monarchy — keep an eye on General Seegers’s daughter, Malu (Saskia Rosendahl), a class traitor who actually is a lefty.

In Season 3, we watched that old guard slowly be supplanted by the Nazi Party, whose paramilitary arm, the brownshirted SA, begins wreaking havoc. A new convert is Helga’s estranged teenage son, Moritz (Ivo Pietzcker), who is closer to his uncle, Gereon (Helga used to be married to Gereon’s brother).

Moritz seems drawn to the Hitler Youth out of loneliness rather than ideology, yet there he is, comfortable with a swastika armband. Whom will he be loyal to in Season 4?

And its opposition

As monarchists, upstart fascists, communists and the governing Social Democrats battled for control of Germany in the first three “Babylon” seasons, several outcomes for the country’s political future were still possible. As the Nazi Party grows in strength, it becomes fractured by internal divisions, while the embattled communists try to provide a revolutionary alternative.

Chief among them is Dr. Völcker (Jördis Triebel), an activist physician fighting both police brutality and the Nazis, while trying to help the poor.

Crime, the old-school way

There are many conceptions of power in the show, and as they attempt to maintain the rule of law, Gereon and Lotte must deal with the Berlin underworld just as much as with the Nazis or the communists.

The kingpin Edgar, known as the Armenian (Misel Maticevic), owns Moka Efti and was a taciturn, subtly threatening presence in the first three seasons. He is close to fellow mobster Walter (Ronald Zehrfeld), who has been having an affair with Edgar’s wife, Esther (Meret Becker). Gangland’s influence over Berliners’ entertainment plays a rather punchy role in Season 4.