@snap2grid Another option might be to put your synopsis in a subtree that has a tag, like this:
* Chapter 1
** Synopsis :syn:
This is a test. It has several lines in it.
This is a test. It has several lines in it.
This is a test. It has several lines in it.
** Body
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
* Chapter 2
** Synopsis :syn:
This is a test. It has several lines in it.
This is a test. It has several lines in it.
This is a test. It has several lines in it.
** Body
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
This is a much longer thing.
Then you'll need the following in your Emacs configuration (typically ~/.config/emacs/init.el or ~/.emacs.d/init.el):
After that, C-c \ (org-match-sparse-tree) with syn as the input should collapse everything except for the subtrees matching :syn:. By default, matching headlines are shown but stay collapsed, which is why we added tags-tree local to the org-fold-show-context-detail variable. (It's org-show-context-detail in Org < 9.6.) If you add more tags, you can even display subsets of the synopses depending on the tags you put in (syn+tag1+tag2...).