Opertoon, which specializes in “stories you can play,” has just released the latest chapter in my favorite story to play, Upgrade Soul.

For months now, I’ve been singing Upgrade Soul’s praises here on AppAdvice and IRL. Indeed, how can I not sing its praises when this wonderful graphic novel app so deserves them?

To echo the pull-quote in the screenshot above, which is taken from my post about the app just prior to its release in late October last year: “If you think you’ve seen everything that can be done on the iOS platform as far as graphic novels are concerned, think again.”

As mentioned, the latest chapter in the Upgrade Soul saga has arrived. The new chapter is the fourth in the projected 18-issue series. In Chapter 4, “Dr. Kallose struggles to placate his troubled sister in the aftermath of the Nonnars’ failed procedure, and a shocking secret is revealed.”

If you can’t see the video embedded above, please click here.

Coinciding with the release of this new chapter is the app’s price drop from $1.99 to nothing at all. In other words, the app can now be downloaded at zero initial cost. That is to say, it is on sale for free.

“The sale is a great opportunity for people who already love Upgrade Soul to share its peculiar brand of immersive storytelling with their friends, and for those who’ve been wondering what the fuss is about to try it for the first time,” said Opertoon founder Erik Loyer.

Note, though, that this free offer is valid for a limited time only. So, if you haven’t already downloaded it, better get Upgrade Soul for your iPhone, iPod touch, or iPad right now.

Chapter 1 is already included in every new download of Upgrade Soul. Additional chapters, up to Chapter 4 for now, can be unlocked via in-app purchase for $1.99 each. Alternatively, Chapters 2 to 6 can be preordered at a 30 percent discount for $6.99.

Also, check out Ruben & Lullaby and Strange Rain, which I recently featured on Shelf Control, if you’re looking for more “stories you can play” from Opertoon.