You play as Corvo, a bodyguard for a Empress. However, she gets murdered by assassins, her daughter Emily is kidnapped, and you are framed. Corvo escapes from jail with the help of a group of rebels, and from there you are sent on assassination missions to defeat the ones responsible and prevent them from taking the throne.
Despite being an assassin, the game tells you to try to avoid killing people, although it's up to you to play how you want. I chose the stealth route, but choosing to only assassinate the main targets (non lethal options are available). The penalty for deaths is that there will be more rats and therefore more Weapers which are zombie-like humans, in addition to a darker ending to the game.
At first I thought the graphics look a bit dated since some textures are pretty poor, but in general, the game does look great and it is surprisingly colourful, given the dystopian setting. The steam-punk and magic aspects of the game reminded me of Bioshock.
If you are playing stealthily, expect to spend a while surveying the area, analysing guard's movements, and plotting your path through the environment, utilising cover and slowly moving whilst crouching. If you approach enemies from behind, you can knock them unconscious then hide the bodies if need be.
Alerted guards will attempt to find you, and guards that are aware of you will attack you with swords and/or pistols, calling in reinforcements to outnumber you. Combat can be fairly difficult if you don't have the appropriate upgrades, but if you do, you can be a complete killing machine. Since I was playing with a stealth approach, I tended to reload when detected, taking advantage of the ability to save anywhere. There's fairly frequent auto-saves too.
There's a few abilities that can help play stealthily. There are some objects you can pick up and throw to use as distractions. You also have the ability to peer around corners without breaking from cover. One of the magic abilities (Dark Vision) is to highlight guards (even through walls) and see their cone of vision.
In terms of weapons, you always have your sword equipped, but then also carry a crossbow, pistol, grenades and traps.
The first magic ability you are given is Blink. This allows you to teleport short distances which is great for traversing the environment, or escaping from danger. It could have made the game too easy, but I think the levels are well designed and it is well implemented. In enclosed spaces, it doesn't provide much use, but in the more open areas, it is great fun zipping from rooftops into an open window, or reaching heights from ground level.
When you are given Blink, you also obtain a strange heart. When held, it shows you how far away the hidden Runes and Bone Charms are. In each area, there can be several of these to find. Runes allow you to unlock magical upgrades, whereas Bone Charms give you a specific bonus. It can be quite a challenge to work out how to get them, so they act as puzzles. E.g., it looked like I could just enter a building through a window, but it turns out the room is boarded up. So you have to work your way around the back of the building and find a path that way.
I had a lot of fun playing Dishonored and I think it would be interesting to replay whilst taking a different approach to the levels.