Wordpress - Search and replace on a post content at view time

I have a plugin that was not created by me, that grabs web content from the web and displays inside a wordpress post.

The problem is that the app description shows the text with no <br \>. The plugin converts everything to \n.

I had an idea of building a one line plugin to intercept the post being written at view time and replacing \n with <br />.

I have never created a plugin for wordpress. Reading the web I come out with this:

<?php 
    /*
    Plugin Name: Convert \n in HTML BR
    Plugin URI: 
    Description: Convert \n in HTML BR
    Author: Me
    Version: 1.0
    Author URI: 
    */

function my_function($id) {
  $the_post = get_post($id);
  $content = str_replace("\n", "<br />", $the_post->post_content);
  return $content;
}
add_action('the_post', 'my_function');

?>

But this has no effect.

I have also tried this:

add_filter('the_content', 'modify_content');
function modify_content($content) {
    global $post;
    if ($post->ID != $desired_id)
        return $content;

    $modified_content = str_replace("\n", "<br />", $the_post->post_content);
    return $modified_content;
}

What is wrong? I am basically following a recipe from a post on the web.

via Chebli Mohamed