Correctly send user to 404 if dynamic content is not found (ASP.NET MVC)

I have implemented 404 handling for the general case in ASP.NET MVC 3, for when a controller/view is not found. But how should it be handled inside the controller if the user is trying to access something that can't be found? For example www.foo.bar/Games/Details/randomjunk will call this inside GamesController:

public ActionResult Details(string id) // id is 'randomjunk'
 {
   if(DoesGameExist(id) == false)
     // Now what?
 

I could just do a return Redirect('/Errors/Http404'); but that doesn't seem like the correct way to do it. Should you throw an exception, or something else?

We could have a special view in this case, but to start with we need a good way we can apply to several cases.

Edit: I want to show my friendly 404 page I already have for the general case.

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