tinchodevel

http://codex.wordpress.org/Template_Tags/get_posts

The problem there, is that you are loading your plugin before the global query has run, so is_single() will always return false.

Try,

function mod_content($content){
if(is_single()){
// do stuff, return content
}
else{
// do nothing, return content
}
}

add_filter(“the_content”, “mod_content”, 10, 1);