"For Good" starts off with Kara waking up from a nightmare. She sees Reign and the Worldkiller we saw at the end of last week's episode. At the DEO, Kara tells Winn and J'onn about the vision in her nightmare.
Mon-El walks in to remind them that while Worldkillers were created on Krypton, the Worldkillers may have been genetically modified to be resistant to kryptonite.
Alex is helping Sam get ready for an MRI, hoping they can diagnose the problem with Sam.
Elsewhere in National City, James Olsen and Lena are discussing a potential date when they bump into Morgan Edge, and the three share a tense moment before Edge drives off.
Edge is driving down the road when the gas pedal sticks and the car powers forward. It appears someone has taken control of the car. Fortunately for Edge, he's able to exit out of the trunk before the car crashes off of a dock.
Upset, Edge storms into CatCo accusing Lena of hacking his car. Edge warns Lena that she better be ready to finish what she started, stating that he "never loses." Edge is removed from the premises and banned from CatCo.
Sam's MRI is finished, and Alex doesn't see anything wrong with the images. Alex tells her that they'll need to send the images to a specialist along with a blood sample. While taking the blood sample, Sam expresses her regret with lying to Lena about skipping work to deal with her personal health.
Alex offers Sam some reassurance, and convinces her to talk to Lena and Kara about what's been going on.
Eve brings in a cup of coffee for Lena, who starts convulsing and foaming
Kara flies Lena to the DEO where Alex finds out she's been poisoned with cyanide. Alex says they need to slow her body down, so Kara uses her ice breath to slow Lena's vitals down, saving her.
Edge is walking to his car when he's approached by James dressed up as Guardian. James begins interrogating him, and Edge denies having anything to do with the poisoning.
J'onn informs Kara that Lena will be waking up soon and suggests Kara take her back to CatCo.
Lena wakes up with James and Kara by her side. She informs the pair that she thought she had a dream of flying through the air with Kara, and they convince her it was
James tells the ladies that the bullet used to kill the hitman was made using cutting-edge technology, and after doing some digging, Lena finds that L-Corp had been developing tech like it. She goes to an L-Corp development site, where her mother is waiting for her.
Lillian reveals that she's the one that killed the hitman that poisoned Lena. She tells Lena that she plans on taking out Edge, but Lena tells her to stay away.
Lena returns to CatCo, where she's promptly approached by Alex and Sam. Sam tells her boss about the personal problems she's been going through. Sam reveals that her biggest fear is leaving Ruby behind on Earth without a support system, to which Kara, Lena, and Alex offer their support.
Lena's out on her balcony when Kara walks in and asks her what she wanted to talk about earlier. Lena confesses that she had previously tried killing Morgan Edge. Lena then tells Kara that her mom is back in town. Lena asks for Kara's help with saving Edge from Lillian.
Lena and Kara go to a gala that Edge is hosting and are able to get past his security. Lena tries to get Edge to confess that he tried killing Lena, and one of Lillian's drones starts shooting at him. Under pressure, Edge admits to everything: the lead poisoning and the poisoning of Lena.
Lillian approaches the two at the gala and is upset that her daughter didn't let her kill Morgan Edge. Lillian transforms the murderous drone into a full suit and is soon approached by Mon-El, who's arrived to help. The two battle with Kara while Edge takes Lena's device that has the recording of him admitting to all wrongdoing.
Edge leads Lena on a foot chase before he's stopped by James, who's suited up as Guardian. Lena gets her device back, and Mon-El and Kara are able to disable Lillian's drone armor.
Edge is arrested, and Lillian is taken away on a gurney.
Alex tells Sam that all of her tests came back clear. No tumors or disease were found. Sam's still upset because she feels something is wrong, and Lena tells Sam to go home and spend time with Ruby.
At the DEO, Mon-El approaches Kara and makes a mention
Winn tells Kara and Mon-El that he's found four leads of potential Worldkillers, and Kara singles out the one named Julia Freeman as the one in her vision.