SAN FRANCISCO – The slow-moving struggle to unplug TV viewing from its traditional business model just got more interesting — and messier.

Over the past few years, companies big and small — from Microsoft and Google to newcomers Roku and Boxee — have tried to bridge the gap between the Internet and the TV to let viewers watch the shows and movies of their choice on the biggest screen in the house. Last week Apple rejoined that effort.

The Cupertino, Calif., company unveiled a lineup of TV show episodes for rent on its iTunes Store. All rent for 99 cents and come in high-definition and without commercials.

To go with that TV inventory — as well as a catalog of movie rentals — Apple introduced an upgraded $99 version of its Apple TV media receiver.

Apple’s updated video vision falls in line with that of such competitors as Amazon’s video-on-demand store and the free, ad-supported viewing available at the websites of the TV networks and Hulu, which is owned by some of them.

All those offerings mean viewers don’t have to pay for things they don’t want to watch.

Music listeners got used to that freedom years ago, thanks to the success of download stores such as iTunes and Amazon’s MP3 store.

But the TV industry has been remarkably successful in resisting that transition. Apple could get only Fox and ABC to sign onto this U.S.-only venture.