“Not Kansas” is episode twenty-one of season three of Supergirl.
In the previous episode, “Dark Side of the Moon”, Kara and Mon-El were given some of the black rock and arrived back just as Reign broke out. They fight Reign as Lena works on the rock. Reign is injected and separates from Sam before evaporating.
Alex brings Ruby to the DEO to see Sam. Supergirl asks Lena if she can synthesise the black rock as well, and explains why. Lena thinks she can.
Back home, Kara tells Alex about her mother being alive, and the city where she grew up still being there. She wants to go back. Alex filled her heart, but Kara doesn’t know how long she will be gone. Alex thinks it’s time for Kara to take care of herself.
The DEO is preparing a goodbye party and Supergirl is speaking to Mon-El. He wants her to see if she can find some tech Argo has so he can send a message to the future for the Legion to come and get him. Supergirl suggests he comes with. He agrees.
Kara and Mon-El return to Argo and are greeted by Alura and Selena. Kara is at her home going through some of her old things when an old friend of hers, Thara, arrives. The chief peace officer. She knows where Mon-El can get a synthetic gravity manipulator when he asks.
Winn and James, as Guardian, are having a slow day when a bank robber is reported. Guardian goes up against the bank robber, who has an unusually effective automatic weapon. Winn wonders what kind of gun that is.
Alex is with J’onn and M’yrnn. M’yrnn has been doing better, but he says it’s the surge of clarity that comes before the end. They need to perform the Reach. A ceremony in which the elder generation passes down their memories, knowledge and experience before they pass. J’onn is in denial that this is happening. Alex gets paged from work and J’onn says he will come with.
Winn and James are looking at a gun and Guardian’s shield when J’onn arrived. The damage was done by a DEO assault rifle. J’onn says that’s impossible, after Winn’s mother walked off with weapons J’onn had them all microchipped. They tried a DEO weapon on another shield; the damage is identical. J’onn says it’s proprietary tech; it could only have come from the manufacturer.
Kara is telling Mon-El she likes being normal and they are talking to a boy Mon-El helped when beams from a construction site fall. Kara sees someone in a hooded robe leaving afterwards. Thara tells her that the construction site hadn’t been inspected. She thinks it was an accident but agrees to look into it.
Kara has never had accidents really, but she starts looking paranoid. However, no-one knows that Selena is connected to the Worldkillers. And it seems doubtful that whatever she did to stop Reign was designed to truly help Supergirl.