You want to add a filter to the filter queue on the_content
:
add_filter('the_content', 'anything');
WordPress core then calls apply_filters
within the_content
function to run all filters in the queue:
function the_content( $more_link_text = null, $strip_teaser = false) {
$content = get_the_content( $more_link_text, $strip_teaser );
$content = apply_filters( 'the_content', $content );
$content = str_replace( ']]>', ']]>', $content );
echo $content;
}