Sometime after 4x10. This takes only the idea of the mind wipe but isn't other wise connected to season four.
Alex Danvers had a long day. Two alien-related incidents needed her attention the third was covered by Colonel Haley. But now she was ready for sisters' night and headed to Kara's apartment. She tried the door handle and it opened. "Why doesn't she ever lock the door?", Alex asked herself.
"Kara, it's me", she shouted as she entered the room while she was carrying to bags with takeout. Suddenly she recognized the smell of smoke and burned things.
"What's that awful smell? Did you try to cook?", she chuckled trying to suppress an uneasy feeling and put the bags on the counter.
"I guess, that's me", a familiar voice said.
Alex turned around and saw James on the couch. His head was lowered and his Guardian suit was spotted with burn holes. Alex felt fear rising in her.
"What did happen to you? … Where's Kara?" Alex asked. She knew something was not right. James rose his view and looked Alex in the eyes. Still he hesitated to say something. She watched him tensed. Finally he cleared his throat.
"We followed a hint. Big story. … big disaster. … Kara is … Kara is gone" he nearly whispered, his voice shaking.
"What do you mean by 'gone'?" Alex asked trembling and furious at the same time. She knew what he meant – his face, his body language left no doubt about it. While her brain tried to make sense of his words she was still standing in the middle of the room.
"Alex, she's dead.", he stammered. "Kara is dead. Supergirl is dead", he repeated as he had to remind himself of this.
"No", Alex said in disbelieve and denial. She made some steps in his direction.
"Yes. … I can't … can't believe it myself. … and I was there. That I made it…" James said looking painful.
Alex made some fast steps and grabbed the remains of his suit. "James, what happened? What happened to Kara?"
"A source told us about an underground enterprise keeping aliens like slaves. A place copying designer clothes. We went there together with Supergirl. We even called J'onn for support. They were indeed manufacturing cheap clothes there but only to cover the shippings of their real goods. Weapons. Based on alien technology and extremly deadly. They were working on artificial kryptonite, too."
"Cadmus?", Alex asked scared.
"Don't know. But it must have been a trap. We found the materials, some caged aliens, others with depowering devices. But no guards. Suddenly there was a huge boom. Everything went yellow and green."
"Kryptonite explosion? The fire at Cinamon Street? ", Alex concluded with a shiver. The incident Haley had covered.
"Yes. Chaos everywhere. I was out for … I don't know. When I woke up there was fire everywhere. I was looking for the others. Couldn't see a thing because of the smoke. I stumbled forwards. Then I found Supergirl. She was smashed under a three-meter-shelf, spiked with Kryptonite and surrounded by green-flames…", he began to tremble by retelling. Alex let go of his suit and sat next to him. She was torn between on the one hand comforting James and giving him the time he needed to tell but on the other hand getting to know the fate of her sister.
"She was coughing blood and shouted with her dying voice at me to get out. … She knew I couldn't lift the shelf and that the fire was spreading to other flammable materials. It was the storeroom for Kryptonite. It must have been a set up. ... Suddenly there was J'onn and I never saw him so afraid."
"The White Martians used fire to kill the green." Alex explained.
"K… Supergirl shouted at him to open the cages and get me out. He was wavering but then did so. Then he ran back in the expanding fire because he did not want to leave someone behind. Then everything blew up", James told her.
Alex eyes widened.
"J'onn … too?"
"No, he got blown out. But he has serious injuries. We brought him to one of Lena's labs. Brainy is helping her to get him stabilised."
"But what happened to Kara?"
"She was between me and J'onn" that was not a lie "She must have been in the centre of the explosion. … We didn't find her. … You know about the heat of a kryptonite fire…"
She didn't know exactly the definite temperature. What James described sounded like an inferno – with her little sister in the centre of it. Denial kicked in.
"But she has not been found yet?" Alex asked almost begging. "They are still working at the crime side"
James head was like a roller coaster. Now, after Kara was gone, should he tell Alex the truth, what she knew before the mind wipe – that Kara and Supergirl were the same person? Was the purpose of Alex' sacrifice gone or should the secret be kept to keep Alex safe?
"No. But there's just dust. Mountains of dust. No human being will get there out alive. Not even Supergirl did", he cried. "I'm so sorry"