Robert Mueller has just landed another big fish in his search for the truth.

The special counsel appointed by the Justice Department to look into the “Russia thing” appears now to have reeled in a three-star witness to help learn a great deal about Donald John Trump’s relationship with the Russian government.

He is retired Army Lt. Gen. Michael Flynn, the one-time national security adviser to the president. Flynn pleaded guilty to lying to the FBI about his conversations with Russian officials. In exchange he has agreed to cooperate with Mueller’s legal team as it pores through a growing pile of evidence.

Mueller already has secured an indictment of former campaign chief Paul Manafort and a chief deputy; former campaign aide George Papadopoulos has copped a guilty plea as well.

Now it’s Flynn’s turn to sing.

As the Washington Post reports: With the guilty plea Friday by former national security adviser Michael Flynn — one of Trump’s closest and most valued aides — the investigation has swept up an array of figures with intimate knowledge of the campaign, the transition and the White House.

It appears to have swiftly expanded beyond Russia’s interference in the campaign to encompass a range of activities, including contacts with Russian officials during the transition and alleged money laundering that took place long before Trump ran for office.

Where does Mueller go from here?

I, of course, am in no position to predict what will happen next, or beyond the next step. My gut — along with my trick knee — are telling me that Mueller’s investigation well might be getting close to pay dirt.

Here’s hoping the president has the good sense to let him stay on the hunt. I mean, Donald Trump keeps saying there’s nothing to any of it … right?