The open source LLMs are really capable, I think the method used to feed the plot might be the more important part in making this work.

You can try setting up Ollama on your RPi, then use a highly-quantized variant of the Mistral model (or quantize it yourself with GGUF+llama.cpp). You can do some very heavy quantization (2-bit), which will increase the error rate. But if you are only planning to use the generated text as a starting point, it might be useful nevertheless. Also see: https://github.com/ollama/ollama/blob/main/docs/import.md#importing-pytorch--safetensors

Here are some pre-quantized variants of Mistral 7B: https://huggingface.co/TheBloke/Mistral-7B-v0.1-GGUF

(all the tools and models I have mentioned in my comment are free and open-source, and beyond that, require no uplink during operation)

I found this blog post were the author tries to use chat GPT to generate theatre manuscript/narrative. It’s based on the paper “Co-Writing Screenplays and Theatre Scripts with Language Models: An Evaluation by Industry Professionals”. In the blog post they outline their narrative generation procedure in this chart:

I also found this GitHub repo with links to more resources on this topic.

Have you tried GPT4All https://gpt4all.io/index.html ? It runs on CPU so is a bit slow, but it’s a way to run various LLM locally with an plug and play, easy to use solution. That said, LLM are huge, and perform better on GPU, provided you have a GPU big enough. Here is the trap. How much do you want to spend in a GPU ?

Are you looking to make an easy buck by generating novels and self publishing them on Amazon?