I tend to avoid stealth games like the plague because quite frankly, I suck at them. I even cringe when I run across a stealth mission in action games or rpgs. Fortunately, Dishonored gives you the choice of playing it either way, however there's a catch and it's called chaos. But more about that later.
You play as Corvo Attano, Royal Protector of the Empress Jessamine Kaldwin, ruler of Dunwall; a fictitious city rememiscent of 17th century London. And just like London, the city is besieged by the plague and overrun by rats. Sent on a diplomatic mission to other cities to discover a cure, you return earlier than expected and interrupt the assassination of the Empress. However, being overpowered by the assassins, you are conveniently - for the plotters - arrested for her murder. At the same time, they kidnap the young daughter, Emily, to prevent her ascension to the throne.
After 6 months in prison, Corvo is about to be executed when he receives help to escape from unknown sources. Once free, he meets up with some Loyalists who want to find Emily and rout the men responsible for the Empress's death. They want you to assassinate the people responsible, find Emily, and re-instate the rightful ruler. Your missions are to target each one and exact revenge. Along the way, you will pick up a side objective, which is optional. Prior to embarking on the missions, Corvo meets the Outsider – a supernatural being who brands him with a mark that enables Corvo to use unnatural powers. Spawned by the Void, the Outsider is said to be neither good nor evil and remains largely neutral concerning human affairs. He does, however, possess an overriding curiosity about what they will do with the powers he gives them.
Although there are not many main missions, they do take a fairly long time to complete and even longer if you decide to stealth it. Non-lethal methods require much more patience and planning. One flaw with this is your lack of non-lethal options when you are spotted. Obviously, you can't use the choke hold as this only works from behind, which leaves using your very limited supply of sleep darts, which can't be upgraded, or running (blinking out of there) and waiting till the guards return to a state of low awareness.
Choosing a lethal or non-lethal route affects your chaos level and therefore the game ending. One is a good ending and one is a bad ending. High chaos may also affect the numbers of the guard, the man-eating rats and the plague wracked weepers. However, low chaos can still be achieved by being discriminating about killing. It's been suggested that keeping this number below 20% will still result in a low chaos ending. An absolute no-kill run provides a lot of challenges as unconscious bodies can be attacked by the rats and die. This counts as a kill. You therefore have to hide them in a safe place. If you are interested in this, you should research what constitutes a kill as I believe any action initiated by you that ultimately leads to a death is counted against you. An example would be deaths due to an explosion or rewiring the electric gates where unsuspecting guards may get fried.
You have several powers at your disposal that can be upgraded to level 1 and 2 by purchasing these with runes that you find along the way. Some powers are not suitable for a non-lethal approach. The Outsider, who appears to Corvo and is believed to be the source of all magic in the world, initially provides you with the means to track these runes. You can also discover bone charms, which give you a slight increase in certain abilities. You can equip a certain number of these and can also upgrade the number of slots. Other items include a crossbow, gun, grenades, traps, rewire tool, mask optics, health vials and mana vials in addition to some defensive upgrades such as boot silence and armor. Upgrades may require you to find blueprints.
I ran into another problem. I found myself naturally playing the stealth route even though I'm not fond of purely stealth games. The reason was three-fold; I wanted to see if I could rise to the challenge of patience, a natural inclination on my part not to kill innocents, and the incentive to get the good ending. So the game sets up this dissonance in your mind because the most fun route is assassination. This speaks to the good immersion in the character as he's not the type to kill indiscriminately and it feels unnatural to do so. Unfortunately, killing is where the most fun is as it opens up many more options in the gameplay. High chaos allows you to utilize all of the tools the game gives you, whereas stealth has only a few tools that are suitable to that role. Someone made an extraordinary video showcasing all the various ways to perform kills, but unfortunately it's been taken down. It hit all the game news sites for a few days and was positively inspiring
If there is a Dishonored 2, I'd like to see a little more balance between the options. The game is very good, however I feel that the non-violent path needs some tweaking to make it a less restrictive and more of a fun experience. Although the game is not really all that long, replay value is high with the two options and for those who wish to try all of the available powers. However, there is no official new game +.
There is a GOTY Edition that includes all of the DLC and in Dishonored's case, they are actually very good and worth playing. Only the Dunwall City Trials, a series of challenge maps, gets mixed reviews. The story DLCs are great.UPDATE Aug 2015: Dishonored 2 is on the way for 2016. You will be able to play as either Corvo or as a grown-up Emily.