Source: Capcom
Since Monster Hunter Stories 2: Wings of Ruin is a JRPG, it has a heavier focus on telling a story than the mainline games. A mainline Monster Hunter game's plot is usually a simple, episodic tale where an unstoppable predator comes by to disrupt the ecosystem, and you must defend your home from it. By contrast, The plot of a Monster Hunter Stories title has you traveling the world to solve a mystery that could impact the entire world and develop friendships with monsters and NPCs along the way.
The setting of Monster Hunter Stories 2: Wings of Ruin has you assume the role of a Monster Rider. Riders strive to preserve the balance of nature by working together with monsters instead of hunting them like Monster Hunters. Although, Riders will slay monsters if they have no other choice when their friends' lives are on the line.