Surprise, surprise - Agents of SHIELD's Advanced Threat Containment Unit might be hiding a major secret.

'Agents Of SHIELD' Season 3: Simmons Wants To Go Where?! Get The Scoop On That Shocking Reveal

It's just so hard to trust people in the post-Hydra world, and the Rosalind's (Constance Zimmer) ATCU task force may prove to be far more malicious than Coulson (Clark Gregg) originally believed. Whatever the case, the truth will come tumbling out when SHIELD has to protect one of their own in episode 6, "Among Us Hide..."

"The stakes get even higher as Hunter (Nick Blood) and May (Ming-Na Wen) continue to go after Ward (Brett Dalton) and Hydra, and Daisy (Chloe Bennet) and Coulson begin to suspect that the ATCU may be keeping a big secret from S.H.I.E.L.D.," ABC teased.

'Agents Of SHIELD' Season 3: Lash's Role Is 'Not What It Seems', Chloe Bennet Explains

The episode will see the debut of Powers Boothe's Gideon Malick, a character who Marvel described as "incredibly menacing" and "a character that even the stone-cold Grant Ward would fear." Considering Marvel never casts a person twice, Boothe will most likely be playing the same World Security Council member he portrayed in The Avengers.

Meanwhile, Lash (Matthew Willig) will make a return. We're still itching to know why the sonic wannabe knows the word "Inhumans" and why he's hell-bent on executing them.

Catch Agents of SHIELD Tuesdays at 9 p.m. ET on ABC. Click the video below to see a preview for the series' next episode, "Devils You Know."

Tags: Marvel's Agents of S.H.I.E.L.D. television ABC Marvel