Robert Mueller has operated for 19 months as a law unto himself, reminding us of the awesome and destructive powers of special counsels. About the only possible check on Mr. Mueller is a judge who is wise to the tricks of prosecutors and investigators. Good news: That’s what we got this week.

Former national security adviser Mike Flynn a year ago pleaded guilty to one count of lying to the Federal Bureau of Investigation about his conversations with Russia’s ambassador to the U.S. Mr. Flynn’s defense team this week filed a...