add_filter() has other 2 optionals parameters (same for add_action()), the priority (default set to 10, and lower numbers correspond with earlier execution) and the number of args (default to 1).
You need to set your filter with the priority with an higher priority than the other plugin, assuming the other has its priority to the default one :
add_filter( 'the_content', 'theme_slug_filter_the_content', 99 );
That all!