SEOUL, SOUTH KOREA — On Saturday evening in Seoul, images of President Moon Jae-in of South Korea embracing North Korea’s Kim Jong-un lit up tens of millions of smartphones. The Presidential Blue House announced that Mr. Moon had just met with Mr. Kim on the northern side of the border — their second encounter in a month. At a press briefing Sunday morning, Mr. Moon explained that Pyongyang had made the request, via the inter-Korean hotline, to speak “informally.”

It was a bold recovery for Mr. Moon, who had been perceived as a tragic middleman since President Trump canceled a planned summit with North Korea last week. That cancellation, made just hours after Mr. Moon left the White House, was received as a national and personal insult. Kim Hong-kook, a journalist and professor at Kyonggi University, described it as a “Trumpian power play” that could weaken Mr. Moon.

South Koreans were thus astonished by Saturday’s secret meeting, which the local press admiringly referred to as “shock theater.” Mr. Moon returned from the North with a message aimed at Washington: “There’s no question that Chairman Kim is willing to denuclearize. But if he does, he wants assurances that the United States will respect North Korea’s sovereignty,” he said.

There are signals that Mr. Trump and Mr. Kim — now with open urging from the South — may go through with their June 12 summit after all.