A plot device is a method (often unrealistic enough that we notice it) used by an author to get from Point A to Point B in his story. In this case (though I don't have my copy of L of R Vol III handy), Frodo and Sam somehow manage to reach the crater's edge, although it is improbable or impossible, because JRR needs to put them there in order to advance his plot-- they are now waiting for Gwaihir the Windlord, perhaps.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.