Full Idea
Plantinga rejects other possible worlds, but adds to our world an uncountable multitude of sets of propositions, each set a way that the world might have been, but is in fact not. (Roughly, for each Lewis world, Plantinga has such a set).
Gist of Idea
Plantinga says there is just this world, with possibilities expressed in propositions
Source
report of Alvin Plantinga (The Nature of Necessity [1974]) by David M. Armstrong - Truth and Truthmakers 07.2
Book Reference
Armstrong,D.M.: 'Truth and Truthmakers' [CUP 2004], p.83
A Reaction
To me it seems as ontologically extravagant to postulate unexpressed propositions as to postulate concrete possible worlds. I think the best line is that there is just the actual world, with the possibilities implied in its dispositions.