Overriding single plugin translation

Here’s an example where a string from a certain text domain is translated using the gettext filter:

/**
 * Translate a certain string from a particular text domain.
 *
 * @param string $translation  Translated text.
 * @param string $text         Text to translate.
 * @param string $domain       Text domain. Unique identifier for retrieving translated strings.
 *
 * @return string
 */
add_filter( 'gettext', 'wpse_translate_string', 10, 3 );
function wpse_translate_string( $translation, $text, $domain ) {
    if ( 'plugin_text_domain' === $domain ) {
        if ( 'Original string...' === $text ) {
            $translation = 'New string!';
        }
    }

    return $translation;
}