SPOILER WARNING: This article contains major spoilers for "Suspicious Minds," the mid-season premiere of Supergirl Season 4.
Kara's relationship with her sister Alex Danvers just got completely upended. In "Suspicious Minds," the mid-season premiere of Supergirl Season 4, Colonel Lauren Haley continued her mission to discover the Girl of Steel's true identity. To prevent this from happening, Alex asked J'onn to wipe her memory so that she forgot Kara is Supergirl.
Even as she dealt with invisible aliens hellbent on revenge, Haley interrogated every member of the DEO in order to learn Supergirl's true identity. Eventually, she broke through when an agent who had promised to keep the secret slipped up. Fortunately, Haley wasn't able to act on that information right away; she was immediately dragged into a fight when the invisible aliens infiltrated the DEO.
During the battle, Haley had a close call with one of the aliens. Supergirl showed up just in time to save Haley, but that didn't soften Haley's demeanor. The colonel immediately blackmailed Kara, demanding that she reenlist with the DEO and follow orders directly or have her secret identity exposed. Alex couldn't stand for that; she immediately decked Haley across the face, knocking her out, and locked her in a cell. Kara and Alex quickly called J'onn in to erase Haley's memory.
Though Haley forgot Kara's identity, she didn't forget her mission. Once the invisible aliens were dealt with, she called in her big gun: an alien with the ability to force the truth out of its victims. In an effort to protect Supergirl, several agents pledged to have their minds wiped by J'onn. At that, Alex revealed that she too would have her mind erased, as Brainiac-5 predicted a 100% chance of Haley finding out if Alex's memory remained intact.
At the very end of the episode, Alex carried through with her promise, but not without plenty of tears. Though Alex will remember her sister, she won't remember that Kara is Supergirl. This will protect Supergirl from Haley, but it may have some unintended consequences. After all, the preview for next week's episode shows that Kara was important to Alex's formative years; by removing Kara's influence, they may have unintentionally changed Alex's personality or priorities. Seeing as Haley is willing to cross lines when it comes to aliens, as evidenced by her willingness to torture aliens, a change in Alex's prerogative could spell disaster for Supergirl and their friends.
Airing Sundays at 8 p.m. ET/PT on The CW, Supergirl stars Melissa Benoist as Kara Danvers, David Harewood as Martian Manhunter, Mehcad Brooks as Jimmy Olsen and Chyler Leigh as Alex Danvers.