Threads is a 1984 post-apocalyptic BBC TV movie. I watched this when it aired on PBS and found it deeply affecting. My first-born was a baby at the time, and watching this bleak film by myself was quite an experience.
AMC has an overview. BBC has a guide and says, "The message - if there is one - that seems to run through the story is that mankind would probably survive in some form or another, but it wouldn't be pretty."