The 'Game of Thrones' season-six premiere was the show's worst, according to critics

HBO's massively successful fantasy drama, "Game of Thrones," returned on Sunday. And while it scored with viewers, critics were a slightly tougher crowd.

According to review site Rotten Tomatoes, the show earned an 81% "fresh" score. That's pretty good, a solid "B" if we were talking school grades.

But the real competition is against itself, right? In this case, the mixed reviews turned in by critics made Sunday's episode the show's worst-scored season premiere.

Here's a look at the show's season premiere scores together:

Season 1 Episode 1 - 100%

Season 2 Episode 1 - 100%

Season 3 Episode 1 - 100%

Season 4 Episode 1 - 97%

Season 5 Episode 1 - 100%

Season 6 Episode 1 - 81%

To extend the classroom comparison, if Sunday's premiere was graded on a curve, it would earn a "D."

Rotten Tomatoes Editor-in-Chief Matt Atchity provided some more context on what critics thought of Sunday's episode and why it earned the score that it did.

"The long-awaited sixth season of 'Game of Thrones' has mostly satisfied critics," he explained. "As the series has outpaced the novels, book readers no longer have the luxury of knowing major plot points ahead of time. Most critics say that the season premiere is a solid return to form, although some say that the abundance of storylines and characters has started to make the series unwieldy."

Thankfully for HBO, "Game of Thrones" has nine more episodes this season to bring its grade up.