My plugin is outputting with weird results

(0.139350) Calling Class/Method: Categoree/categoree
(0.139381) Nested Plugins in Tag, Parsing Outward First
(0.000040) Tag: {exp:gallery:entries gallery="photos" category="{cat_id}
(0.000254) Closing Tag Found
(0.000347) Module Tag: Gallery/entries</code></pre>

Here is the relevant template code:
<pre><code>{exp:categoree parse="inwards"}
    cat is {cat_id}
    {exp:gallery:entries gallery="{gallery_name}" category="{cat_id}" orderby="random" columns="1" rows="1"}
        {image_url}
    {/exp:gallery:entries}
{/exp:categoree}

function Categoree()
    {
        global $DB, $IN, $TMPL, $FNS;
        
        $cat_id = $TMPL->fetch_param('cat_id');
        
        if ( $cat_id == '' )
        {
            if ( ! preg_match("#(^|\/)C(\d+)#", $IN->QSTR, $match))
            {    
                return '';
            }
            
            $cat_id = $match['2'];
        }

        $query = $DB->query("SELECT exp_gallery_categories.cat_id, exp_gallery_categories.cat_name
                FROM exp_gallery_categories, exp_galleries
                WHERE exp_gallery_categories.gallery_id = exp_galleries.gallery_id AND 
                exp_gallery_categories.parent_id = '".$DB->escape_str($cat_id)."'");

        if ($query->num_rows == 0)
        {
            return '';
        }
        
        $tagdata = '';
        
        foreach ( $query->result as $row )
        {
            $tagdata .= $FNS->prep_conditionals($TMPL->tagdata, $row);

            foreach ($TMPL->var_single as $key => $val)
            {
                switch ($val)
                {
                    case "cat_name":
                        $tagdata = $TMPL->swap_var_single($val, $row['cat_name'], $tagdata);
                        break;
                    case "cat_id":
                        $tagdata = $TMPL->swap_var_single($val, $row['cat_id'], $tagdata);
                        break;
                }
            }
        }
        $this->return_data = $tagdata;
    }