Special Counsel Robert Mueller has dispatched the Russia collusion theories to the land of cranks and bitter-enders, but there are still some loose ends. To wit, how did the partisan propaganda known as the Steele dossier become the basis for an unprecedented FBI probe of a presidential campaign, an abuse of law enforcement, and two years of media and political hysteria?

We know the outlines of the story but not all the details, and no one has yet been held accountable under the law or even held to much public scrutiny. It’s...