Every action has an equal and opposite reaction. Newton’s third law describes bodies in motion, but it can also be applied to human interaction. Each act of love can be reciprocated by love. Conversely, violent acts — though they can appear one-sided — reverberate and rebound.

That’s the physics that describes “The Last of Us Part II.” It’s a sequel that brings a reckoning for the violence in the original.

That game’s climax put players in a horrific position. Joel brought Ellie to the Fireflies, a militia hellbent on finding a cure to a brain infection that has consumed the world. Ellie is immune to the disease, which essentially turns people into zombies. She is the key to saving the world. The only problem is that the cure would kill her, and Joel makes the fateful decision to murder the militia to save the child.

The sequel follows the aftermath of that choice and it acts as a fulcrum for the game. Ellie and Joel settle in the town of Jackson, Wyo. They live a quiet life until an attack by unknown assailants changes their lives and slingshots Ellie and other town members to Seattle for retaliation.

“The Last of Us Part II” is Ellie’s story and a study of revenge. The missions, broken up into several days, show developer Naughty Dog pushing the limits of its level design, creating expansive environments that contain nooks and crannies of exploration throughout. Most of the combat is based on stealth, as Ellie picks apart squads of adversaries by crawling through tall grass and shivving them. When stealth is broken, gunfights erupt and it becomes about outflanking enemies and efficiently killing them with weapons at hand.

Ellie kills her adversaries with a frightful viciousness, but a strange thing happens in “The Last of Us Part II.” The campaign flips the script and it forces empathy on players. They realize that they are not killing faceless enemies but people dealing with their own plights.

“It’s a story of tribalism; of how we vilify and dehumanize those on the outside,” says director Neil Druckmann. “When is it time to let go? And when should you hold on at any cost? This is a story of trauma, redemption and empathy.”