Adding Templates

I'm an experienced PHP programmer, but fairly new to Drupal, so I still haven't fully wrapped by head around the theming flow.

I have a custom module that declares a new node type 'floorplan'. The module is basically complete and I have everything working. However, when viewing the node I want to use an external phptemplate file for the content. node.tpl.php is already customized to my liking, but I want to use a separate file for the $content area. The reason is that I'm importing an old template that is poorly written with tons of nested tables and extraneous HTML. I don't want to bloat up my module with all this cruft, and I don't have time to clean up the HTML. So I created a new template file called floorplan_details.tpl.php. Then I have defined hook_view as follows:

function floorplan_view(&$node, $teaser = FALSE, $page = FALSE, $block = FALSE) {
  $output = '';

  $output .= theme('floorplan_details');

  $node->body = $node->teaser = $output;
}

I don't have a theme_floorplan_floorplan_details() function, because I just want it to go straight to the template. However, the output of theme('floorplan_details'); is empty. There are also no errors being raised. What am I missing?