If I were to die and someone found my notebook with my stories then tried to continue them, could they have the same plot as me or is that impossible?

I don't know what you're doing, but the answer to this is in copyright law. Which means it also depends on which country you and your notebooks were in.

The vast majority of countries are signatories to the Berne Convention, which means that as soon as your stories were fixed in a tangible medium, the notebooks, they were covered by copyright. So check on your country.

Assuming your country is a signatory, when you died, that copyright remained in force. For how long depends again on country. In the US, UK and many others, the copyright is in force for 70 years past your death. Other countries range from 50 to 100 years.

Last point. With your death, ownership of your copyright passes to your heir as part of your estate.

Which means… if that someone is your heir, then yes, they're free to create derivative works based on your notebooks and continue the stories, as they own the copyright.

If that someone is not your heir, then no, they cannot create derivative works. Unless they negotiate a licensing deal with your heir to allow them to create the derivative works.

If your country is not a Berne signatory, then you'll have to check local copyright laws.