“For Good” is episode twelve of season three of Supergirl.
Kara wakes from a dream about Reign and other Worldkillers, one clearly the second Worldkiller revealed in the previous episode, “Fort Rozz”.
At the DEO, the only Kryptonians they are tracking are Kara and her cousin. However, Mon-El says the Worldkillers might have been created on Krypton, but they are not strictly Kryptonian. They left Krypton before it exploded, and have possibly been waiting for Reign. Mon-El says that not just pods came from Krypton, but Kryptonite as well. J’onn asks Winn to check meteor showers for that time. They wonder what Reign is doing. Probably planning her next move, according to Kara.
In fact, Sam is having an MRI done at L-Corp by Alex. Sam is worried that her memory lapses could be caused by a brain tumour or ALS. Alex tells her caffeine withdrawal also causes them.
Lena and James are talking and heading for breakfast when they bump into Morgan Edge. The meeting does not go well. Edge drives off, then something takes control of his car. He manages to tumble out through the boot as it leaps off the road and explodes over the river.
At CatCo, they’re talking about news stories when Edge throws his keys. Kara catches them. Edge is a mess and accuses Lena of hacking his car and trying to kill him. He says he will finish what she started. Afterwards, Lena says that was an uncharacteristic outburst for Edge. And it’s pointed out Lena never retaliated for Edge trying to kill her. Except she did.
The MRI has revealed nothing, but Alex is going to send the images, and bloodwork, to an expert. Sam is tired of lying to Lena. She doesn’t want to become a burden. Alex reassures her she’s sick, not a burden. Sam is used to doing everything for herself since she was 16. Alex is similar, but she explains how Kara wouldn’t let her withdraw into herself after Maggie left, and Alex won’t let Sam nearly make the same mistake. She has friends.
NCPD has found no sign of tampering on Edge’s car, and they don’t think Edge did it himself to frame Lena. Eve brings Lena a coffee, which she drinks. And then collapses. Kara says it’s poison; she’ll take care of Lena. James asks Eve where the coffee came from. The cart, as usual. James heads there to find the person who was on it running, he chases, they shoot at him. And then the assassin is shot by someone else.
Kara takes Lena to the DEO. Alex manages to identify the poison as cyanide. They need to slow down Lena’s circulation by inducing hypothermia. Fortunately, they have someone with really cold breath.
Though it’s likely Edge tried to poison Lena, even if he naturally won’t admit it, Lena doesn’t seem to be the one who tried to kill him. Meaning someone else did. And it looks like Sam doesn’t show as vaguely Kryptonian most of the time.