Warning: major spoilers ahead for Episode 10 of Agents of S.H.I.E.L.D. Season 4.
Agents of S.H.I.E.L.D. has always touched on the events of major Marvel movies, and its closest ties over the years have been to Captain America's films. After all, the events of Captain America: The Winter Soldier totally blew up the show's premise and forced a narrative reboot in its very first season, and Captain America: Civil War was pretty impactful in Season 3. The latest episode of S.H.I.E.L.D. went for a major blast from the past via a connection to the 1940s events of Captain America: The First Avenger, as General Talbot dropped the bombshell that everybody's favorite Inhuman director of S.H.I.E.L.D. is not what he seems, saying this:
That's right. Director Jeffrey Mace, the hero of Vienna who bravely revealed his status as an Inhuman to the world is nothing more than a fraud juiced up on a version of a super soldier serum courtesy of the mysterious Project Patriot. General Talbot and the President evidently came to the conclusion that S.H.I.E.L.D. needed a way to recover its image after the end of Season 3 and gain the trust of the Inhumans, and the way to do that was to create their own Inhuman and prop him up as the director. Jeffrey Mace is a man who happened to have the right bureaucratic background and was willing to fake enhanced abilities. Building a brave and bold American hero to boost morale worked in the 1940s with Cap; why shouldn't it work in 2017 with Mace?
Of course, General Talbot was not entirely successful in finding a Captain America 2.0. Unlike Cap, Director Mace requires injections at regular intervals to maintain his enhanced strength. Mace has been taking the injections on the sly during his entire tenure at S.H.I.E.L.D, deceiving his co-workers and the public alike. Talbot may have succeeded in building a patriot, but Mace is certainly no Cap.
Both the Marvel Cinematic Universe and Agents of S.H.I.E.L.D. have made the point in the past that failure followed all attempts to recreate the exact serum that transformed the diminutive Steve Rogers into the magnificent physical specimen that is Captain America. S.H.I.E.L.D., however, went in a different direction to give Mace his abilities as a fake Inhuman, and General Talbot and Co. refined the formula that transformed Daisy's dad into an aggressive monstrosity back in Season 2. The government scientists removed the elements that drove Cal Johnson mad and created a serum that worked well enough to grant super strength for a brief window of time. Project Patriot created something that could provide the powers of Cal without all the crazy.
And Talbot would have gotten away with it if not for those meddlin' kidnappers who nabbed Mace before he could re-juice himself back to Inhuman status! Coulson did not react well to the reveal that Mace had been faking his identity as an Inhuman, but he wasn't so angry that he wanted Mace gone from S.H.I.E.L.D. Instead, Coulson has decided to keep Mace on board as a figurehead while retaking control of the agency himself. Coulson will be calling the shots at S.H.I.E.L.D. once more; the public just won't know about it.