Output the field as $_rendered in the node page
To load the data back to the node and render them, you first need to implement a hook_nodeapi load. Here some a short example:

function satya_module_nodeapi(&amp;$node, $op, $teaser, $page) {
  switch ($op) {
    case 'load':
      // retrieve the p1 value from custom_table (it's better to use vid instead of nid to allow revisions.
      $p1 = db_result(db_query('SELECT p1 FROM {custom_table} WHERE vid = %d', $node-&gt;vid));
      return array('p1' =&gt; $p1);
      break;
  }
}


Then you will have a $p1 variable available in your node.tpl.php.

If you don't know yet the Examples modules, I'll definitely recommends it to have great examples and developer documentation for a lot of hooks.