There is the_content() in one of the template files which, well, prints the content. I get the content with get_the_content() manipulate it and print it.
However, I couldn’t figure out which action or filter to use to prevent the_content() being called in the template file (single-post.php). I do have child template, but for some reasons, I don’t want to manipulate the template file for single posts.
Read more here:: Removing the_content() from the page