Calling plugin code/function from Custom Template Page

I’m trying to create a Printable Calendar from a Calendar Plugin. I’ve created a “Custom Template” page (“CustomTemplate.php”). That Custom Template file needs to be located in the directory for the theme. I will use it to create a calendar on a separate page without the WordPress Header and Footer (and including some event text that is normally visible only when the user’s mouse hover’s over the event title). I need to have code in this php file that calls functions that are defined in a file located in the plugin directory (“../calendar”).

What’s the right way to solve this problem? Normally, a page that includes the calendar would simply include this tag:
[Calendar]

…but I need to write my own special functions that will print out the calendar in a printable format so I can’t use the exact same function that would normally be called by [Calendar]. How should I do this?

Thanks in advance for any help…