WARNING: The following contains major spoilers for "A Secret Kept From All the Rest," the penultimate episode of Batwoman Season 1.
The key to killing Batwoman lies in a frequent Supergirl plot device: Kryptonite.
In "A Secret Kept From All the Rest," the penultimate episode of Season 1, Alice finally got her hands on a translation of Lucius Fox's journal and quickly realized why he went to such great lengths to keep its contents secret. As it turns out, the journal contains instructions on how to kill Batman. That means Alice now has a way to kill Batwoman.
Throughout the episode, Alice searched high and low for a way to decode the journal. Eventually, this led to Hush kidnapping Julia Pennyworth and Luke Fox. Alice forced Luke to translate the journal by threatening to kill Julia. He managed to crack the code, but refused to translate it for fear that she would kill Kate and dismantle Batman's legacy.
When Alice moved to make good on her promise to kill Julia, Kate arrived and intervened. She offered Alice a pair of Lucius' glasses, which have the ability to translate the journal, in exchange for Luke's safety. The swap was made, but not before Alice opened all the cells in Arkham Asylum, hoping the ensuing chaos would lead to Batwoman's arrest. However, this also forced Alice and Mouse to go on the run again, much to Mouse's dismay.
"This isn't a shelter, Alice. This is back where we started," Mouse told Alice when they made it safely to the sewers. "I told you that inviting Luke into your obsession would draw out the Bat, and now here we are, freezing, wet, drenched in sewer water."
"And I told you that Luke would bring us the answers that we need, so I suppose we're both right," Alice mused.
"I was happy, Alice. I was happy. What happened to you? Do you even care that you burnt the whole thing down?!" he cried.
"I care about killing the Bat," Alice spat back. "So go bring me that walking facial gone wrong [Hush] so he can tell me where to find a little green rock called Kryptonite."
At the time of writing, it is unclear how Kryptonite factors into Lucius' instructions to kill Batman. However, it must be an important part of the process, or Alice wouldn't want it. Notably, this also appears to be part of the fallout from "Crisis on Infinite Earths," which merged the main Arrowverse (Earth-1) with Supergirl's universe.
"I've talked about this before, but the only way I was really comfortable in doing the crossover was if we could find ways to keep our world still feeling really grounded. One of the most wonderful aspects of the crossover was the Kate Kane-Kara Danvers friendship," showrunner Caroline Dries told EW. "Part of that storyline was centered around this piece of Kryptonite, and it just kind of represented this bond between them; it was just so much bigger than alien mineral [because] it represented trust and a rhyme to Bruce Wayne and Clark Kent’s relationship from the comics. So, I wanted to keep that in our world. Even though it’s an alien rock, we have introduced it on our show."
"It was just something [where I thought], we've seen high tech stuff -- like we’ve talked about a rail gun and a coil accelerator -- but is there something that actually could pierce the suit that is not just some weird piece of technology that somebody else could recreate? Is there something unique? So I was like, 'Yes, Kryptonite!' There are very few [rocks] in our ether," she added.
As Batwoman heads into its Season 1 finale, Alice will ramp up her efforts to kill Batwoman. However, her critical decision to run from the relative safety of Arkham might land her in some hot water before she can get her hands on some Kryptonite. After all, according to the synopsis for the episode, "Alice has uncovered what could finally take down Batwoman, but she is losing hold on her henchman Mouse and Hush, sending her spiraling into her most wicked self."
Created by Caroline Dries and developed by Berlanti Productions and Warner Bros. Television, Batwoman stars Ruby Rose, Rachel Skarsten, Meagan Tandy, Camrus Johnson, Dougray Scott, Elizabeth Anweis and Nicole Kang. The series airs Sundays at 8 p.m. ET/PT on The CW.