How can I call plugin function from a custom template?

I’ve just developed a calendar plugin.
To display a calendar in a template, I call do_shortcode(args...);.

This works fine.

But how can I call a function from my plugin that will return data?

I don’t want to use include_once – I really just want to call a function E.g.

$events = calendar->getEvents(args...)

As long as the plugin is active, the function will work perfectly fine from any template.