WHAT IT’S ABOUT In this alternate-history series based on Philip K. Dick’s 1962 novel of the same name, Nazi Germany and Japan have won World War II. They’ve invaded the United States — after Washington is destroyed in a German nuclear strike — and divided the country up between themselves. In San Francisco, Juliana Crain (Alexa Davalos) has been handed a mysterious film canister by her sister and told to deliver it to the “man in the high castle.” Boarding a bus, she heads to “neutral” territory in Nevada, where she meets Joe Blake (Luke Kleintank). He’s from Nazi-occupied New York and wants to help.

Meanwhile, back East, the head of Nazi America, John Smith (Rufus Sewell), would also like to learn a little more about this “man in the high castle.” His counterpart in the west, Chief Inspector Kido (Joel de la Fuente), does as well. They also both want that film — which offers an alternate history of its own.

The premiere episode streamed last January; 10 episodes will start streaming Friday.

MY SAY Aside from “Doctor Who,” TV has never exactly embraced the “alternate history” genre, and reasons why aren’t clear. Maybe real history is good (or bad) enough, already offering a vast expanse of canvas on which to play with facts and stories. Or maybe show runners and the networks that hire them have longer memories than we usually give them credit for. In the ’80s, ABC’s movie “The Day After” and follow-up alternate-history mini, “Amerika,” were seen by millions of viewers, and also bummed out every one of them. Bumming out viewers as an operating principle has rarely been embraced by the TV networks, at least on purpose, so leave it to a streaming service to bum them out instead.

And, yup, “Castle” can be a bummer, an unrelenting one. Of necessity, “The Man in the High Castle” is dark — so dark that your first impulse might be to reach for a flashlight to pierce the gloom. In triumph, the Nazis of “High Castle” haven’t temporized their most barbaric impulses but merely perfected them. By the early ’60s (when this is set), Zyklon B is odorless, even painless. Jets fly from Berlin to the West Coast in hours, which apparently means that the V-2 rocket program was useful after all. In this alternate reality, the Nazis did develop a nuclear weapon, even though real history tells us Hitler could have not cared less about “atoms” and what they could do.

But most dispiriting are Americans themselves. From their ranks, the Nazis have found Quislings and fellow travelers, like Sewell’s blandly named “John Smith” with the monstrous honorific, “Obergruppenfuhrer.” He and his perverse “Leave It to Beaver” brood even live on a tree-lined street, in a nice neighborhood, identified only as “Greater Nazi Reich, Eastern Division, Long Island.”

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In theory, Amazon’s “High Castle” should be full of ideas — big, swelling, exciting ones, about the nature of freedom, and the role of individuals in a democracy; or about fate, or the tenuous nature of human existence. In fact, it’s not entirely clear (from the first four hours I saw) that “Castle” is much interested in those ideas, or at least putting them before the story — a story that proceeds deliberatively, by the way, often too deliberatively.

But bummer or not, this mini is at least different. It offers something you’ve never really seen before, and those ideas — if they ever arrive — might even make this 10-hour journey worthwhile. Just don’t be too surprised (or disappointed) if you have to get through 10 hours to find out.

BOTTOM LINE Intriguing . . . but somber and slowww-moving.