The finale finds Simone realizing that she has no other choice but to try and kill her own brother, Rasmus, who could be the final hurdle to getting a cure. She needs to inject him with the virus. Unfortunately, her initial plans fail when Rasmus makes one of his soldiers walk into Simone’s trap first. However, Sarah. Rasmus’ girlfriend, shows up and makes Rasmus have a change of heart.
Rasmus and Sarah end up sacrificing themselves by feeding the main flower (the cure), creating an explosion that kills anyone with the virus in the vicinity. Simone and the rest of the group head out into the world to hopefully spread the cure and save what’s left of humanity.
In the closing monologue, Simone tributes her brother:
“You saved the world. It was you. My little brother. I was supposed to protect you. That was the plan all along: making sure that you’d never be alone. Instead, I am. All alone,” she explained. “I wish I could show you the world of tomorrow. You gave us hope. We’re here thanks to you. And we’re rebuilding the world in your honor. You are in everything that I do, in my every thought. You are forever my sweetheart. I love you.”
Source : https://www.distractify.com/p/does-martin-die-the-rain