Protesters hold missing-person notices for booksellers with the Mighty Current publishing house in Hong Kong on Jan. 3. (Anthony Wallace/Agence France-Presse via Getty Images)

THE STEELY president of China, Xi Jinping, was quite possibly shaken up when he read book titles being peddled in Hong Kong lately. One was “The Collapse of Xi Jinping in 2017.” Another was “The Great Depression of 2017.” A forthcoming title was reported to describe Mr. Xi’s love life before he became president. The books came from Mighty Current, a publisher specializing in volumes that are highly critical of China’s Communist Party leaders and often filled with lurid and poorly sourced gossip. If Mr. Xi didn’t see the books personally, his security henchmen certainly could not have failed to notice.

In recent months, something happened to five men who led and worked at the publishing house. On Oct. 17, Gui Minhai, the owner, who had a Swedish passport, vanished from his beachfront apartment in Thailand. Then three others associated with the publisher disappeared while visiting the mainland Chinese cities of Dongguan and Shenzhen. Most recently, Lee Bo, a business partner of Mr. Gui, disappeared on Dec. 30; according to Mr. Lee’s wife, he was lured to a Hong Kong warehouse where the publisher stores its books and did not return.

So far, there is no solid information about what happened to the five. But all signs point to Chinese state security officers kidnapping and detaining them. Some of those who disappeared have made brief and furtive phone calls to family members, never saying where they are or why they disappeared. If the suspicions are true, then the case of the Hong Kong booksellers marks another chapter in China’s brazen campaign to silence critics where they live — anywhere.

Hong Kong, a former British colony, was turned over to China in 1997 with an explicit promise that it could preserve its cherished freedoms under the principle of “one country, two systems,” meaning that China’s rulers, who tolerate little dissent at home, would not interfere with its freewheeling capitalism or democracy. If security thugs from the party-state in Beijing are now snatching Hong Kong booksellers who publish critical words about China’s leaders, then the pledge of 1997 is in tatters. The case of the missing booksellers seems to be another bell tolling for Hong Kong’s freedoms and autonomy.

Just as ominous is the specter of the Chinese state abducting people abroad, such as the disappearance of Mr. Gui in Thailand. We have expressed alarm previously about China’s aggressive snatching of dissidents and their families overseas, including the kidnapping of Wang Bingzhang, a democracy activist who was abducted while on a trip to Vietnam in 2002 and then sentenced to life in prison in China. In October, China reached across the border into Burma to nab the fleeing 16-year-old son of human rights lawyers who were detained in July.

It’s one thing to have patience with China on the complex issues of trade and maritime conflict. But there can be no patience with this behavior, in which China’s ugly quest for total control is being imposed far beyond the mainland.