From the time “Mister Roger’s Neighborhood” first appeared in 1968, to its final episode more than 30 years later, it seemed like the quintessential feel-good children’s TV show. The genial, soft-spoken host Fred Rogers invited his young viewers into his make-believe neighborhood, introduced them to his kindly friends like Mr. McFeely and Neighbor Aber and entertained the kids with old-fashioned puppets. It could hardly be more innocuous.

So what was Mr. Rogers doing addressing hot-button topics like nuclear war and race relations?

The show proved groundbreaking because Rogers showed a fearlessness in tackling topics that many people would have considered taboo, especially for children. And as history would show, Rogers was often ahead of his time.

Rogers—who had planned to be a Presbyterian minister—frequently covered simple, somewhat silly topics that were relatable for children—such as why they shouldn’t fear getting a haircut, or how they were too large to go down the bathroom drain. But Rogers believed children could handle discussing much more complex topics, especially if they were addressed gently and honestly in a way that made youngsters feel safe.

In 1968, just four years after the passing of the 1964 Civil Rights Act that ended legal discrimination, and in the same year as the assassination of Dr. Martin Luther King, Jr., Rogers cast François Clemmons, who was African American, as Officer Clemmons on his show. This part made Clemmons the first black actor to have a recurring role on a children’s show.

One year later, the program included a memorable scene in which Rogers was resting his feet in a kiddie pool, and he invited Clemmons to put his feet in as well. This act was viewed by many as an explicit gesture against racism during this difficult period of integration.

In an interview, Clemmons said he was skeptical of playing a black policeman. In the neighborhood where he grew up, law-enforcement officers were less-than-friendly figures. But he said the pool scene touched him in ways he was not prepared for: “The icon Fred Rogers was not only showing my brown skin in the tub with his white skin as two friends. But as I was getting out of that tub, he was helping dry my feet.”

And as he was toweling off his friend, Mr. Rogers told his young viewers, “Sometimes a minute like this will really make a difference.” While on the surface, he was talking about cooling off on a hot day, Clemmons knew it went far deeper than that: “He was making a very strong statement.”

In 1981, Rogers talked about another challenging subject on his show: Divorce. As broken marriages became more common in the country, Rogers attempted to communicate to children that their parents’ marital woes were in no way caused by them. To deliver this message, the show had the postman, Mr. McFeely, essentially run off camera when the topic of divorce was brought up, to show how even adults can be uncomfortable with the topic. Rogers then broached the topic with children who were watching from home.

That same year, “Mister Rogers’ Neighborhood” had a visitor who would become among its most famous: 10-year-old Jeffrey Erlanger. Erlanger, a quadriplegic, showed Rogers his electric wheelchair and how it worked, while also explaining how having a spinal tumor at a young age had rendered him paralyzed in his arms and legs. Nearly a decade before the passing of the Americans with Disabilities Act in 1990, the segment aimed to humanize individuals with disabilities, and show how they adapted.

In 1983, as the United States and the Soviet Union were at peak Cold War tension, Rogers talked about a particularly tough topic for children: nuclear war. In five episodes called “the conflict series,” Rogers used his puppets to tell the story of how one of them, King Friday, who ruled “The Neighborhood of Make-Believe,” was anxious that another kingdom was building bombs. Defensively, King Friday orders the people of his land to build bombs as well.

Ultimately, the series reveals that the rival kingdom is actually communicating an anti-arms race message. Turns out, they were building a bridge, not bombs.