Saving happens by itself

There is no save button. Plot Device saves a moment after you stop typing, and at once when you switch chapters, leave the window, or close the app.

The save status

The top bar shows where things stand:

It says	Which means
Saved	Everything you wrote is on disk.
Saving…	Writing it now.
Not saved yet	You just typed; it saves in a moment.
Changed in another app	The chapter changed outside Plot Device. Nothing was overwritten: see When a file changes in another app.

If saving ever fails, for example because the disk is full, your text stays in Plot Device and saving tries again as you type.

Safe writes

Every save writes a complete new copy of the file, then swaps it in, so a crash or power cut never leaves half a chapter behind. And before Plot Device changes a file, it keeps the old one: see Earlier versions.