Where is this in default theme?

Using the default theme, on the main page, when you click on the title of a post you are taken to a page with just that post, with no sidebar. I like this clean look. However, the theme I am working with takes you to that page, but includes the sidebar. I want this theme to display a clean simple page like the default does.

Where do I look to edit out the display of the sidebar for the individual post page? I just can’t see what is calling the sidebar to display on that page.

Thanks.