In a story which takes place between the Lord of the Rings trilogy and the Hobbit, you play as a Ranger called Talion who after being killed by Sauron's armies, is resurrected with Wraith-like abilities and goes on a quest into Mordor to exact his revenge.
Roam around an open world, taking on side quests and missions at your own pace and as either a Ranger or a Wraith, priding either stealth or combat. Shadow of Mordor also introduces the Nemesis System, where Talion's interactions with specific characters may change enemy behaviours or how certain characters react to him. Be wary of each interaction and build your Talion the way you want.