Spoiler alert: NBC’s “This Is Us” is one of the biggest hits of the fall season.

And while on the subject of spoilers, here’s another alert: If you’re brand-new to “Us,” which is about the Pearson family in the past and in the present, but have every intention of watching when you have the time, then it’s best to avoid this story for the moment. What follows is a spoiler attempt to explain why one show has hit the sweet spot for 17-plus million viewers every week.

There are lots of reasons behind the success of “Us,” which is about Jack (Milo Ventimiglia) and Rebecca Pearson (Mandy Moore), who adopt a black baby the day their own pair of twins are born. (Their third baby was stillborn.) Here are some obvious ones:

1. IT’S COUNTERPROGRAMMING There’s no sex, no murder, no gunplay nor explosions nor mayhem nor the walking dead nor terrorists nor procedural-based cops who wrap the story by whatever violent means necessary after the last commercial break. Instead, “Us” is a prime-time anomaly. Put another way, what’s a nice show like “Us” doing in a place like this? There’s nothing else like “Us” on prime time, which has been overrun with guns and violence, forcing millions of viewers to reach for the Zoloft before they reach for the remote. “Us” makes no demands on viewers’ already frazzled TV-induced emotional state. This is a warm seltzer bath.

2. IT’S A HOPEFUL VISION OF “POST-RACIAL” AMERICA An unapologetic sentimentalist of a show, “Us” is also a card-carrying Utopian idealist. Belying — or defying — the reality of modern America and the “Black Lives Matter” movement, “Us” wants to imagine an America where skin color is irrelevant, where black and white live in perfect harmony, where the sins of the past are erased in an enlightened future. After that recent election, maybe viewers were ready for a little optimism.

3. IT’S A SENSITIVE VIEW OF RACE “Us” may be an optimist, but not necessarily a naive one. In fact, the best episode so far, “The Pool” (Oct. 18), managed to explore race with an unusual degree of sensitivity for prime time. In this episode, the Pearsons head to a public pool on a hot summer day. A few black families congregate in one corner of the swim park, with whites everywhere else. Rebecca notices that her adopted black son, Randall (played by Lonnie Chavis; Sterling K. Brown is the adult Randall) has gone missing. He’s playing with the other black kids — one of Randall’s earliest memories of his racial “otherness,” and the beginning of his acute awareness that Jack is not his real father. In present time, Randall is at a performance of his daughter Tess’ play. When Tess (Eris Baker) introduces herself as Snow White, the audience titters, and Randall wilts. As he later snaps at his real father, William Hill (Ron Cephas Jones), “You don’t think I live in a black man’s world? There are a million things every day that I have to choose to let go just so I’m not [expletive] pissed-off all the time.”

4. IT’S A CLEVER INVERSION OF PREJUDICE Randall’s adoptive sister, Kate (Chrissy Metz) and adoptive brother Kevin (Justin Hartley) confront their own forms of prejudice — Kate, because of her weight, and Kevin, because everyone thinks he’s just a pretty, talentless dim bulb. But “Us” has carefully built their interior worlds, which are layered, nuanced and sympathetic. By empathizing (or identifying) with either, viewers — white ones — can also empathize (and identify) with Randall. “Us” actually inverts the discussion of race. Rather than tell viewers what it’s like to be black in America, viewers can actually begin to imagine how prejudice feels.

5. IT’S FUNNY Sorry: These preceding reasons are all a little heavy, and they overlook a key “Us” selling point: Humor arrives suddenly, like a welcome breeze when least expected. Examples abound. Here’s an obvious one: An infuriated Olivia Maine (Janet Montgomery) — the lead of the New York play Kevin ineptly reads for — upon learning he has been hired as her co-lead, says, “They [the producers] evidently think ‘The Nanny’ will sell tickets.” (Kevin’s old sitcom was of course “The Manny.”)

6. IT’S ABOUT REAL PEOPLE WITH REAL WEIGHT ISSUES Some 68.8 percent of American adults are overweight or obese (according to the National Institute of Diabetes and Digestive and Kidney Diseases) but some 0.05 percent of scripted American television features overweight people. TV is filled with thin, pretty people (Kevin) but the insight of “Us” is that those who are beset with weight issues (Kate) are more relatable. This isn’t a sitcom (“Mike & Molly”) but a prime-time drama that looks at the guilt — and anguish — of obesity.

7. IT’S DRIVEN BY A BIG IDEA (OR TWO) By toggling between past and present — the Pearsons as a young family, and the Pearsons 20 years later — that big idea, or ideas, begin to take shape: “Us” is about seizing the moment, and the relentless push of time, and the deep intimacy of past with present. Kevin (in “The Game Plan”) hints at an even bigger idea. Presenting his painting — a swirl of colors — to his nieces, he explains: “Life is full of color and we add our own color [but] what if we’re all in the painting everywhere? What if we were in the painting before we were born and after we die and these colors just keep getting added on top of one another. ... There’s no you or me, it’s just us. We’re just one thing, one painting.” Hey, not bad for the former “manny.”