Metal

They built the machines to destroy each other, but the machines destroyed them
When The Machines took over the world Brett joined the Resistance to fight them. He fought them until he lost everything he cared about and found that he had no more to give.
The only thing he could do then was survive in one of the communes that had made a home in the tunnels beneath the city. He was content and they were happy to leave him alone, until the woman got sick.
Now the community need Brett's unique knowledge and experience. He is a vital member of the rag-tag group that has volunteered to go above ground in search of vital medicine.
But The Machines are waiting for them. Unless the group can find a way to work together the Machines will kill them and they will never find the medicine.