The story stack is a game analysis tool first presented by Jesse Schell in a GDC Talk (his part begins at 51:00.) Ever since I learned about it from Susan O'Connor in The Narrative Department Masterclass, I can't help but use it to analyze every game I play.
At a high level, the story stack is a list of elements of a game, ordered from least flexible (player fantasy) to most flexible (story). I'll explain the roles of each of these elements as I walk through their implementation in Ori and the Blind Forest by Moon Studios.
Player Fantasy
Least flexible in the stack is player fantasy, which is essentially the core player experience. It's what the player expects to be and do when they play the game. All other elements in the story stack should serve the player fantasy.
In Ori and the Blind Forest, the player fantasy is to explore and save a dying forest.
Action
Next in the stack is the actions that the player takes in order to achieve that fantasy. Actions that don't reinforce the fantasy can feel extraneous, and omitting actions that are deeply tied to the fantasy will leave that fantasy unfulfilled for the player.
Since Ori and the Blind Forest's fantasy features exploration, the player's available actions are largely related to traversal, including running, jumping, climbing, and swimming. The player must also fight enemies corrupted by the forest and search for and collect life sources scattered across the game.
Economy
Economy is a bit of a misnomer, because although it can refer to the financial system in a game, it also refers to any kind of progression that the player makes and the results of that progression. This progression should be achieved through taking the above actions and guide the player towards fulfilling the fantasy.
The progression in Ori and the Blind Forest can be broken into large scale and small scale achievements. The big picture progression involves finding and collecting the three Elements of Light and returning them to their proper place in the forest, which is what tangibly brings the character closer to their fantasy of saving the forest. But on a smaller scale, the player progresses by collecting new skills to traverse to new areas, finding energy cells to gain more health, and earning ability points to upgrade skills on a skill tree.
All of these items and power-ups, both big and small, are hidden deep in the world and surrounded by enemies, requiring the player to use their traversal and fighting actions to obtain them.
World
World, as one might expect, is the setting in which the game takes place. A good game setting should be artfully created in order to support all the previous elements in the story stack.
The world for Ori and the Blind Forest is implied by the player fantasy: a dying forest. The game ties this setting into its progression by including different biomes of the forest that the player progresses through, which each represent a different way that the world is struggling.
For example, in the first third of the game, the player explores several areas that are either dried up and rotted or contain toxic pools of water. After the player restores the element of water, the clean water flows freely and transforms the biome, allowing the player to explore new areas by swimming in the fresh water.
Story
The final and most flexible element on the story stack is, in fact, story. This can be unintuitive for writers, but it's essential to remember in order to effectively write for games. If an action or a progression system or an element of the world changes during production, it's the writer's job to change the story to fit, not the other way around.
In Ori and the Blind Forest, the player controls Ori, a long-lost child of the Spirit Tree in the forest of Nibel. After a great cataclysm in the forest, the Spirit Tree used the last of its light to revive Ori, and the elements of the forest were thrown off balance. Now Ori must find these elements and restore them to their proper place. This premise is especially strong because it gives an emotional hook to the main progression system (restoring the Elements of Light.) The player character isn't simply a nobody who decided to help. The elements fell out of balance because the Spirit Tree used all its power to save the PC from death.
Partway through the game, we come across Kuro, a large black owl, who serves as the game's antagonist and tries to keep Ori from saving the Elements of Light. But as we expand our knowledge of the world and the events that led to its downfall, we see that (spoilers!) Kuro is traumatized by the loss of her eggs, and all her actions are to protect her one remaining egg in the only way she knows how.
Defeating Kuro at the end of the game is not a matter of fighting her, but of showing her that you're hurting just like she is. This reinforces the player fantasy, because the player saves not only the forest but also its traumatized residents. However, it creates some ludonarrative dissonance with one of the main actions in the game: fighting corrupted creatures. Kuro attacks Ori just like the hundreds of other enemies that the player kills throughout the game, but there's no narrative explanation for why Kuro gets redeemed while all the others are simply killed.
New Story Concepts
Since story is the most flexible element of the story stack, I challenged myself to create three alternative stories for Ori and the Blind Forest, while keeping the rest of the story stack intact.
Concept 1: Pollution is the Bad Guy
A young tree spirit returns home after running away to the human world and discovers their forest destroyed by pollution. With all of their family now missing, the spirit takes it upon themself to seek out and awaken the three ancient gods of the forest, with hopes that they will aid in restoring their home. Along the way, as they fight pollution spirits and learn more about their source, they discover that their departure from the forest is what tore a rift between their world and the human world, causing the pollution to leech in.
Concept 2: A Child's Imagination
A young boy collects trash in the forest near his house, but to him, it's so much more. We play the game through the lens of his imagination, battling trash monsters and collecting pine cone power-ups. As the game progresses, the player begins to see that this game of make-believe is the boy's escape from a difficult home life, and the adventures he goes on in the forest empower him to face the struggles in his real life.
Concept 3: Witch Blight
After a mysterious blight corrupts all of her crops, a farmer sets out into the nearby woods to discover its source. Along the way, she fights corrupted plants and discovers ancient seeds that allow her to harness the power of plants for herself. As she clears the blight out of the forest, more takes its place, and she discovers the source: three rival witches in a yearslong feud who are cursing each others' gardens. The witches each believe that the others stole their signature arcane artifact, so the player must search the woods for each artifact and return them to the witches in order to stop the blight.