Each Season consists of several different episodes (usually, 3 episodes in total). Each episode requires a separate Pass, so in order to collect the locked rewards from all the episodes, you will need to buy a separate Pass for each episode (three Passes in total).
Compared to the regular Premium Episode Pass, the Premium Episode Pass Plus gives you more unique episode rewards, a Star Booster, and allows you to complete the next 3 tiers at once (i.e. unlock the next 3 rewards on the progress bar), but it still doesn't unlock all the episodes in the Season - it only unlocks the rewards from the specific episode you bought it for.
Also note that in order to get the rewards for Episode 2, you must first purchase a Pass for Episode 1 and then a Pass for Episode 2.