Thief is a single player game focussed around stealth gameplay and problem solving. The story is divided between eight chapters or missions, each of which are bookended by plot exposition either in the form of voiceover or cutscene rendered using the in-game cutscene. At times the audio sync for these scenes is off a little or may be abruptly ended, but the general standard is reasonable given that it's not pre-rendered.
Difficulty is determined by choosing one of four options at the start: Rogue, Thief, Master or Custom. Rogue, Thief and Master simply determine the AI quality and cost of resources, whilst Custom allows the adjustment of each setting individually as well as enabling/disabling saves and other challenging criteria. Enabling Custom settings allows you to post times and scores for missions and client jobs/heists on a leaderboard, competing with other players the world over.
Whilst on a mission or client job you can return to town at any time through the menu, resetting any progress you may have made in that mission. Unless you have the option disabled through a custom setting you can save at almost any time on the PC (so long as you aren't in a spot where you may alert guards). These two options together make progression straight-forward, although at times returning to a vendor to stock up on arrows and consumables can be frustrating due to patrols.
Through challenge modes you can return to specific missions and complete them using more difficult options such as a time restriction or no-detection clause. Results can then be shared with your friends or placed on a wider leaderboard. A couple of challenge modes are currently available, with more to be unlocked as DLC in the future.
In-game movements are governed by typical WASD, but there's one glaring omission - a jump button; instead [SPACE] is used as a one-size-fits-all 'jump up', 'climb up', 'jump over', 'jump across small gap' and the like. There's no means to jump and hang on to a ledge, cross chasms which at first glance look unpassable. As such every path you have is one clearly prescribed by the developers rather than novel or original, making Garrett seem clumsy or at least lacking in the nimbleness you would expect. It really is an odd decision, and one wonders if it was intended to fit a console sensibility or merely to preserve off-limits sections of the in-game world.
Given you spend most of you time creeping around the game could become quite plodding. An injection of pace is provided by the dash skill, which all you to rush for a few steps in your direction of travel. Completely silent, it allows you to skip between shadows or quickly move out of line of sight when necessary. Be warned however, if you dash through a bright area you're still visible, can can be detected quite easily.
Lock Picking and other mini-games.
As a thief you are of course expected to pick the odd lock. Chests, small caches of loot and doors can each have locks of varying difficult, both in number of tumblers you have set in position and sensitivity for each. Generally though solving them doesn't test skill in isolation, but is a tense experience if you're hurrying to unlock it before a patrol returns.
Whilst not requiring skill, nor does the mechanic frustrate in the way similar mechanics have in other titles. Skyrim for example is notorious for both feather-sensitive master locks and consumable lockpicks which would be pretty inappropriate for Thief. Other example in-game mechanics such as finding hidden switches behind paintings are similarly more time rather than skill-testing, with the exception of specific locks such as the Barons Vault.
Focus is an upgradable ability which highlights points of interest in your vision with a blue or red aura. Blue typically refers to loot, consumables, locks and collectsables; red on the other hand almost exclusively indicates traps. It may also point you to locations of interest such as wire boxes or environmental features which you can interact with to disable traps or distract guards. In this respect it's pretty useful, although can be toned down with custom difficulty options.
Staying in focus consumes the focus meter, which is replenished by the consumption of poppies. Even if the meter is empty you're still given a small pulse of focus, allowing you to spot point of interest in your immediate line of sight. Managing focus is key to staying safe and spotting all the available loot.
In the way it operates it's directly comparable to a similar mechanic from the recent Tomb Raider, although far more critical to in-game progression. As in Tomb Raider it's easy to over-use it, breaking immersion to the detriment of the overall experience. Higher difficulties make it a much more scarce resource requiring careful husbanding.
Although it was initially planned, Thief doesn't have have experience system; instead character progression is primarily through unlockable upgrades. Eight categories each with two tiers gradually improve one facet or your stealth, thievery or combat abilities requiring the investment of one Focus Point. Focus points are acquired by certain in-game events or by purchasing them from the Queen of the Beggers for gradually increasing amounts of gold (starting at 150g on Thief difficulty).
Whilst an important gold sink and gold of completionists none of the upgrades are strictly required to progress in the game. By shaving off small amounts of difficulty they make your life slightly easier, but equally those pushing for a challenge can ignore them entirely. Despite their marginal value they nonetheless provide another set of goals which could be seen to be expected in a modern game.
An additional upgrade system is through the Shady Dealer just outside the Crippled Burrick. He sells one-off augmentations to your backpack, armour, slow-fall capabilities and effectiveness of health and poppy consumables. For a price. Once again not necessary for progression in-game, they nonetheless easy the difficulty for a rather steep cost in your swiftly growing horde of gold.
Items and Consumables
Closely associated with character upgrades are items, consumables and item upgrades which can be found around The City or bought from Dealers in town. The most critical are the wrench, wire cutters and razor which are unlocked at some point after Chapter 2 in the story. The wrench allows access through covered culverts and vents, whilst the wire-cutters disable alarms and some locks on doors and windows. The razor allows you to cut out valuable paintings to add to your own collection.
Other items are consumables used throughout the game and well worth keeping topped up. Arrows allow you to interact physically with your environment, or lethally with your enemies where necessary. Some are more useful than others but at the very least ensuring you have plenty of blunt heads and rope arrows before you set out on a mission or heist is sensible. Food and Poppies refill your health and focus bar, and you can stockpile a number at a time determined by your difficulty setting. Finally throwables can be used to distract guards, and if possible you should keep at least one on your person at all times. Thankfully there's no complicated inventory management to process as loot is auto-sold when acquired, eliminating some tedious complexity present in games with stronger RPG elements.
You're certainly not overloaded with items and most have an important role to play, but it is very easy to overlook entire inventory of the Shady Dealer. There's very little signposting that the wrench et al are available and where to find them, which may mean significant confusion for some players.