In some anime, after a year or so of not airing, they usually try to ease its audience back into the series, whether it’s through the form of a filler episode, a special episode, or a couple of episodes or so of no action, drama, and such. Then there are anime that just went straight to the point expecting its audience to already know the series without being eased into it. Dr. Stone is the latter.
It tried to ease us back into the series at the beginning of the first episode. Still, it mainly serves to update the gang’s new member, Ryuusei, on what has happened while he was petrified. Even if it’s to update Ryuusei, it works well as a small refresher of what happened in previous seasons. Soon after, though (right after the season’s first opening), the series went straight and dived right into the story. So, don’t expect to be eased into the series if you haven’t started watching it yet.