Spoilers ahead for Episode 15 of Supergirl Season 3, called "In Search of Lost Time."
Supergirl has been a wild ride throughout Season 3, and the good guys are facing danger on more than one front. On top of the regular villains of the week who turn up to cause trouble, Sam is barely containing Reign, and it's likely only a matter of time before Reign manages to break out of Lena's lab. Throw in the third Worldkiller, known as Pestilence, whose arrival is clearly imminent based on the birds falling dead out of the sky at the end of the episode, and there's plenty of trouble for Kara and Co. in National City. Unfortunately for them, "In Search of Lost Time" may have revealed that Supergirl will need to kill a major character off in Season 3, and that character is none other than Sam.
"In Search of Lost Time" finally delivered some answers about how Sam could have lived her whole life as a powerful Worldkiller while still suffering from all the regular human frailties of illness and injury. According to the tests run by Lena, Sam changes on a cellular level every time she blacks out and becomes Reign. Sam isn't just going through mental breaks when she blacks out; her cells are literally going through metamorphosis. Even if Sam has been able to pass for human with all those human frailties, that may not be the case forever, especially if her blackouts continue.
Lena decided to provoke Sam until Sam blacked out and became Reign since video evidence was the only way Sam would believe that she was really the Worldkiller who had killed so many people and done so much damage. It didn't actually take too much for Sam to turn this time around, and Lena would have been killed if not for precautions. When Sam/Reign inevitably gets out of Lena's lab/lair, she likely won't have much in the way of stopping her. Sure, Kara and Mon-El are working on a strategy to outwit Reign since Kara can't really outmuscle her, but Reign is still a major threat.
Sadly, the fact that Sam and Reign are physically connected means that the good guys probably won't be able to just slap a psychic dampener on her wrist or something similar to keep from going full Reign. Reign isn't just a state of mind; Sam is physically becoming Reign. To stop Reign, Supergirl and Co. may need to kill Sam. Supergirl executive producer Robert Rovner had this to say to EW about the Reign/Sam connection moving forward:
Supergirl, the D.E.O., Lena will all want to help Sam rather than kill her. The Legion of Super-Heroes might have a less sentimental take on the situation, but there are almost certainly more people that want to save Sam than sacrifice her as collateral damage at this point. There's also the point that Supergirl is arguably the lightest of the four Arrow-verse shows, and killing off Sam might fit more on Arrow or Legends of Tomorrow or even The Flash. Still, we shouldn't rule Sam's death out. By the end of Season 3, Supergirl might need to kill off Sam. Ruby could always go live with Alex, right? The show has already scored a renewal for Season 4.
You can catch new episodes of Supergirl on Mondays at 8 p.m. ET on The CW. For more important info on the superhero front, swing by our rundown of important superhero TV dates. If you need to work out what to watch once the 2017-2018 TV season comes to an end, check out our summer TV premiere guide.