A streaming Web site, pictured on a computer screen in Beijing, offers a description of the American TV show "The Big Bang Theory," but it no longer has access to episodes in the series. (Ng Han Guan/Associated Press)

You've probably heard of the "The Big Bang Theory." It's a prime-time CBS sitcom about four dorky guys who befriend an attractive waitress. The show is usually quite family-friendly and is very popular – according to one report from earlier this year, it might be the most popular show in the world.

The show is not known for courting controversy – in fact, the most controversial thing about it is probably whether it is any good (My vote: No, it's not). But, somehow, the show has stepped into a controversy in China – and no one knows quite what to make of it.

Over the weekend, Chinese Internet users noticed that they could no longer view "The Big Bang Theory" online. The show was one of four U.S. television shows (along with the "The Good Wife," "The Practice" and "NCIS") that could no longer be streamed on online video sites such as Sohu. "Sorry, the video is currently unavailable due to policy reasons," read an explanation on the sites.

In some ways, the removal of the show is not so surprising. China is in the middle of a crackdown on pornography and other unsavory behavior online. The campaign, titled "Cleaning the Web" by the Chinese state, has seen some unorthodox targets, such as the e-books section of Sina.com.cn.

But "The Big Bang Theory" is hardly a scandalous show. On IMDB's parental guide, it receives a rating of 15 out of 50. And what's really odd about the online TV show crackdown isn't what they chose to take offline – it's what they allowed to remain on.

For example, "House of Cards" is still available online. That Netflix political drama, very popular in China, has plenty of scenes of violence and sex. And Sunday, China's CCTV began airing a translated version of "Game of Thrones," a fantasy show that frequently shocks even hardened U.S. viewers with profanity, nudity and violence.

So far, China's State Administration of Press, Publication, Radio, Film and Television (SAPPRFT) has refused to comment on the move, but the state newspaper People's Daily appeared to offer an explanation Monday "If you don't have Internet order, how can you have Internet freedom?" the paper said in a commentary, according to the Reuters news agency. "Anyone enjoying and exercising their Internet rights and freedoms must not harm the public interest and cannot violate laws and regulations and public ethics."

Given the lack of statement from SAPPRFT, it's hard to ascertain exactly what laws and regulations might have been violated. A reporter from the official Xinhua News Agency investigated, and it appeared that the copyrights for the shows were all in order.

One theory is that the show was taken down in a bid to bolster China's state broadcasters, who are facing an unprecedented threat from the rise of video-sharing sites. Indeed, the independent Chinese outlet Caijing reported Monday that CCTV is planning to buy the show and subject it to a more rigorous censorship and translation. Perhaps not incidentally, shares of Sohu sank to a one-year low Monday.

Whatever the reasoning, the show's removal has prompted an outpouring of grief from fans. On micro-blogging Web sites, some users posted candle icons (meaning R.I.P.) next to the news, while others criticized censorship encroaching on their viewing habits: “I feel more and more strongly that I am living in North Korea," wrote one user on a microblogging site. Some even suggested after the show was blocked that they wanted to save money to move abroad.

The impassioned response may show another factor in the censorship of the "The Big Bang Theory" — its remarkable popularity. The show racked up more than a billion views in China. As Liz Carter noted over at Tea Leaf Nation in January, a show about male college graduates unable to succeed professionally or romantically apparently resonated with a lot of Chinese viewers.

Hallie Gu in Beijing contributed to this post.

A previous version of this post identified "House of Cards" as a show on HBO. This version has been corrected.