Warning: The following contains SPOILERS for Batwoman season 1, episode 8, "A Mad Tea Party."
The mid-season finale of Batwoman ended with a major character's gruesome death as the result of Alice's machinations. This facilitated a major shift of the show's status quo and big changes when the show continues its story after the Crisis on Infinite Earths event.
The central plot of Batwoman has focused upon the crimes of a costumed mastermind known as Alice and her Wonderland Gang, and how Kate Kane became Batwoman in order to save Gotham City from Alice and her schemes. It was revealed early on that Alice was Beth Kane - Kate Kane's long-lost twin sister, who was presumed dead 15 years earlier. As the season progressed, it turned out that Beth became Alice after years of psychological torture at the hands of a mad doctor who wanted a friend for his hideously scarred son. It was also revealed that Beth's father Jacob Kane had almost rescued her from her captor, but ultimately gave up the search for his missing daughter because of evidence falsified by Catherine Hamilton; a weapons magnate who later became Jacob's second wife and formed the Crows Security firm with him.
Over the past few episodes, Alice had hinted at planning a "mad tea party" where she would finally get her revenge on those who wronged her. Alice put her plan into motion at a gala where Catherine Hamilton was being honored as Gotham City's Humanitarian of the Year. As Catherine was giving her acceptance speech, her teleprompter began to deliver a message from Alice, telling her to keep reading the message or the Wonderland Gang would make sure that her speech "would be to die for." This led to Catherine reading a confession that she had become rich secretly selling weapons to Gotham City's criminals and that her husband had become rich off of people's fears of those same criminals. As Catherine reached the end of Alice's remarks, blood began to run from her nose.
Ushered into a safe room by a member of the Wonderland Gang posing as a security guard, Catherine's daughter Mary, a medical student, began trying to diagnose her mother's apparent injury. Alice arrived shortly thereafter, revealing that she had poisoned Catherine with Project 4782; a nasty neurotoxin developed by Catherine's company that caused dizzy spells, nosebleeds, muscle paralysis, respiratory failure and, eventually, the complete liquidation of the victim's internal organs. Thankfully, Alice had an antidote that she was willing to hand over in exchange for a sincere apology for having faked the DNA evidence that ended the manhunt for Beth Kane.
A desperate Catherine choked out an apology and true to her word, Alice handed over the antidote. Unfortunately, it was at that moment that Mary's nose began to bleed and Alice revealed that she had administered the poison through the complementary champagne in the limousine that took them both to the gala and there was only one dose of antidote. This left Catherine ordering Mary to take the antidote for herself, even as she died a slow, lingering death. To add insult to injury, Alice planted the poison vial on an unconscious Jacob Kane, effectively framing him for the murder of the wife whom he was in the middle of divorcing, who just exposed his apparent criminal activity.
While it seems unlikely that Alice's frame-job will stick for long since Mary is still alive to testify that Alice was the real poisoner, Catherine's death is sure to have a major effect on Gotham City and the story of Batwoman. Over the course of the season, Kate ignored her stepmother and stepsister while trying to reach the Beth she hoped was still somewhere inside Alice. With Alice's latest actions, Kate has given up all hope of saving her sister and realized what she has lost in pushing away Mary, who now wants nothing to do with Kate for choosing the maniac who killed her mother over her. It has also driven another barrier between Kate and her father, leaving Jacob Kane more determined than ever to kill Alice.