Lemme try and explain this, it's a pretty difficult concept.
Okay, let's start with episode 1. Imagine the number one. Now take a deep breath, take your time, what number comes after one?
Correct me if I'm wrong, but I think it's 2. Now for the complex math, after episode 2, came episode 3.
The next episode was them episode 4, followed by episode 5, which was then succeeded by episode 6.
After that, came episode 7, and the next in the series was episode 8. After 8, we have 9, then 10, followed by 11.
Hang in there, we're almost done.
After 11, it was episode 12, then it was lucky number 13, after which episode 14 was released. Then came episode 15, the current one, and next will be 16 in the year 2069.
Okay sorry, but actually, each new episode adds more content onto the end of the last episode, picking up where it left off, HOWEVER, episode 15 is really just the 15th rendition of the game, it includes episode 15 AND the previous 14. Your save file will be retained when you update to episode 16, but you may need to save in a certain area as some maps may change slightly. Also, some events that occurred previously change with new episodes. Ghastly used to be available from the current event for misdreavus, and misdreavus had a separate event which was removed.