Quick recap
Kara shuts down after losing Mon-El. Samantha Arias, Lena's friend and a single mom, discovers she is Reign, a Kryptonian Worldkiller built to conquer Earth. Mon-El returns from the 31st century with the Legion and with a wife, Imra. Reign nearly kills Supergirl and awakens other Worldkillers, Purity and Pestilence. Lena secretly makes Kryptonite. Kara learns that Argo, a Kryptonian city, survived, and that her mother, Alura, is alive. In the end, Sam defeats Reign and is freed from her, and J'onn's father merges with the Earth to save it. J'onn leaves the DEO, and Alex becomes director. Mon-El and Winn leave for the future, and Brainiac 5 stays. In Siberia, a copy of Kara appears.
No spoilersOpens once you mark the title as seen.