“Battles Lost and Won” is episode twenty-three of season three of Supergirl and the season finale.
In the previous episode, “Make It Reign”, Reign flew into the Earth’s core to start turning the planet into a new Krypton.
Supergirl, Alura, Guardian, Alex and Mon-El are helping in National City as J’onn and M’yrnn head to save the planet, along with Imra and Brainy who arrived back in the Legion ship. M’yrnn has only time to give J’onn one memory, that of the first Keeper of the Sacred Scrolls. The moment when she received them. Then M’yrnn heads into the planet and the disruptions stop.
J’onn returns to the DEO and Imra says they got the message from the time beacon. It worked, even though it broke. Sam is still asleep and she’s in the dark valley, where Patricia greets her. Selena and the other two witches are trying to find Reign, as their cauldron is frozen. And Colville isn’t as dead as they thought.
Imra finds Mon-El; she says time apart made her realise that their marriage was never what it should be.
Sam finds the fountain; there are two wells and she goes to drink from one. Patricia says it’s the wrong well and will kill her. All that remains of Patricia is her love for Sam and Sam has to believe her. Sam does, and drinks from the water Patricia hands her.
Brainy finds Mon-El and is concerned he’s smiling. Then Winn comes in and Brainy says this involves him as well. The Legion needs them both. Killing Pestilence saved the future from the Blight. Millions of people were saved. Including a distant relative of Brainy’s. The evil one. He created a plague that will wipe out all AIs other than himself. Brainy can no longer stay; the future needs heroes. It needs Mon-El. And Winn. Winn to take Brainy’s place in the Legion to help save the AIs. Winn doesn’t think he’s any good. Brainy has an artefact from the future. A piece of paper on which Winn drew something for Demos. The design is the foundation of the future’s most ground-breaking tech.
Reign flies out of the cauldron, as Colville eyes J’onn’s ship.
Alex is talking to Lena about having children. Lena reassures her.
Winn tells James, who reassures him he is a genius. Winn does want to go to the future, but his family is here. James explains how he took off his mask and how it helped.
Then an alarm goes off. Colville has triggered the distress signal on J’onn’s ship. They know where Selena is. And they can kill Reign. Supergirl doesn’t want to; Alura and J’onn tell her they might have to.
Lots of people have to decide what to do. And there are new threads leading into season four.