The CW has released a synopsis and 13 images for the upcoming 21st episode of Supergirl season 3. Earlier this week they released images for the next episode, 'Darkside of the Moon', which will air Monday, May 28th, so now they're getting a full episode ahead on images. Spoilers ahead.
Here is the synopsis for the episode called 'Not Kansas':
KARA CONSIDERS A MAJOR LIFE CHANGE — Kara (Melissa Benoist) makes a major life decision. Meanwhile, J'onn (David Harewood) finds out that special DEO-caliber guns have hit the streets of National City. Dermott Downs directed the episode written by Gabriel Llanas & Anna Musky-Goldwyn (#321). Original airdate 6/4/2018.
This seems to be a bit of an odd synopsis for the third-to-last episode of the season. Normally by this point the buildup to the finale starts and everything is focused on the big bad. But here they introduce a storyline about DEO guns getting out on the streets. This could be a side-effect of the changes made on the writing staff after the firing of Andrew Kreisberg earlier in the season and the changes made during the hiatus. You'd have to guess that the major life decision Kara is contemplating has to do with there being a city of Kryptonians floating through space and whether she wants to stay in Argo City with her mother.
As for what we can learn form the images… not much. It seems that James (Mehcad Brooks) is some how involved in the gun story; possibly he's the one that discovers that the DEO-caliber weapons are out there. Other than that we have what looks to be Kara (Melissa Benoist) and Mon-El (Chris Wood) squaring off against something and Lena (Katie McGrath) working in her lab. No images of Reign, Alura, or anything else.
Supergirl airs Mondays at 8 p.m. ET on The CW. 'Not Kansas' will air on June 4th.