WARNING: The following contains spoilers for "The Last Temptation of Barry Allen, Pt. 2," the midseason finale of The Flash, Season 6.
In "The Last Temptation of Barry Allen, Pt. 1," Ramsey Rosso, now nearing his full powers as Bloodwork, worked to take control of Barry Allen. It took a lot of convincing, but eventually, the supervillain was able to transform the Scarlet Speedster into one of his drones as the Negative Flash. Before his turn was revealed, however, Barry infiltrated S.T.A.R. Labs on a mission for Ramsey. Once Iris realized that something was wrong with her husband, the evil Flash sprang into action, attacking his friends and running to his master.
With his powers and his person corrupted by Bloodwork, Barry is no longer the hero we know him to be -- he is now the Negative Flash, and he uses his powers to enforce Ramsey's commands. Now, "The Last Temptation of Barry Allen, Pt. 2" picks up right where the previous episode left off. Cisco and Iris wake up in their lab, following the evil Barry's attack on them. Instantly, Cisco activates a security system comic book fans will know is inspired by Batman: the Babel Protocols.
Upon waking up from the Flash's attack, Cisco runs to a console where he rapidly activates the Babel Protocols. Then, we see an impenetrable energy barrier manifest itself all around the S.T.A.R. Labs building. But the barrier isn't mean to stop any villain from entering the lab. No, it's specifically designed to keep the Flash out. As Cisco explains it, he created this protocol specifically for if (or when) the Flash would ever turn dark. And, as we can see throughout the episode, the barrier truly does its job, keeping the Speed Force-powered Barry Allen out of S.T.A.R. Labs for as long as the protocols are active.
If all of that sounds familiar, it's because it's a nod to the comics. In fact, the entire idea is a reference to Mark Waid and Howard Porter's JLA story "Tower of Babel." This storyline is famous for being the first one that revealed Batman had extensive plans to take out all members of the Justice League, should they ever turn evil. In the comic, those plans were unfortunately stolen by the League of Assassins, who used them to defeat the world's greatest superheroes.
Therefore, Cisco's "Babel Protocols" is a direct reference to this JLA story. Not only does the protocols' name come directly from the comic, but it also shows that, like Batman, Cisco has planned for the worst. He knows how to take out the Flash... but there are other metahumans on his team. Does he have similar plans for Killer Frost and Elongated Man? We wouldn't bet against it.
Airing Tuesdays at 8 p.m. on The CW, The Flash stars Grant Gustin, Candice Patton, Carlos Valdes, Danielle Panabaker, Tom Cavanagh, Jesse L. Martin, Danielle Nicolet and Hartley Sawyer.