The penultimate episode of Firefly Lane season 2 ended with a flash forward to the mysterious 2016 wedding. Kate (Sarah Chalke) is there all there dressed up, and there’s a brief moment where it looks like she may have beat cancer.
The series finale begins in 2016, and Marah is getting married. Kate is still at the wedding, but she’s just a part of Tully’s imagination. “Everything good that she is… is because of you,” Tully (Katherine Heigl) tells Kate about Marah (Yael Yurman). Tully continues, “She is the best of you.” Kate’s fate has been revealed. She’s dead.
Back in 2006, Kate’s cancer has now spread to her brain, which disqualifies her from the trial Danny got her into. Johnny (Ben Lawson) tells Tully that Kate only has a matter of weeks or months if they’re lucky. Tully is in disbelief. She thinks Johnny is giving up on Kate.
Tully goes to Kate’s room. Kate has accepted her fate. She knows she’s dying. “It’s not giving up. It’s letting go,” Kate says. All Kate wants is to go home. She notices that Tully is a little closed off. She knows Tully is upset. “I promised you I would never leave and now I am,” Kate says.
That’s when the dam breaks for Tully. “You can’t die. I don’t know who I am without you,” Tully cries. “I don’t know how to exist without you.” Tully and Kate have been through so much together, Tully hasn’t fully wrapped her head around losing Kate and what that means.
At home, Kate finishes her book. It’s the story of her life. She tells Johnny that she wrote it for Marah so she can remember her. Kate knows that these last few years will be what Marah remembers the most, and she wants Marah to know everything.
Meanwhile, Danny reveals to Tully that he’s moving to New York for a big work opportunity. Tully has been busy taking care of Kate, so she hasn’t been able to talk with Danny too much. Tully, realizing she can’t lose someone else she loves, professes her love for Danny. She asks him to stay, and that’s all he’s ever wanted from her.
Tully runs to Kate’s house to tell her everything about what happened with Danny. They’re sitting out on the porch overlooking the water. Tully leaves Kate alone on the porch while she goes to refill her tea. Kate looks at her surroundings and takes in her last moments. She dies completely content with everything that’s happened in her life. Tully comes outside and finds Kate dead.
When Tully arrives for Kate’s funeral, she admits to Johnny that she doesn’t think she can go in. Kate knew this would happen, so she planned for it. Johnny gives Tully the box that Kate prepped for her funeral. “She left something for you,” Johnny says.
One last gift from her best friend. Kate has written a letter for Tully. She’s given Tully a copy of her book as well, which tells their story from its start. “There’s so many things I should say to you at this moment, but in our lifetime we’ve probably said them all,” Kate writes.
Kate continues, “I know that you’ll be thinking that I left you, but it’s not true. All you have to do is open this book, and you’ll find me.” Tully goes through the box and smokes the joint. She pulls out the picture of David Cassidy and kisses it. She grabs the iPod and plays it. Tully gets up and starts dancing to ABBA’s “Dancing Queen” outside the church. Exactly as Kate wanted. Kate may be gone, but she and Tully will be Firefly Lane girls forever.