Thief is a reboot of the Thief series and throws away most of the lore – there are no Pagans, no Hammers and no Watchers. The game starts when Garrett, a master thief, is on a job with his former apprentice Erin. They run into a mysterious ritual from a faction loosely reminiscent on the Mechanists, and due to unfortunate happenings Erin falls right into the middle of it. Garrett blacks out for a year and awakens in the city which changed to a fascist regime in the meantime. Now Garrett has to use his thieving skills in order to find out what is going on (the common people are affected by a strange disease) and eventually stop the usual conspiracy.
The basic structure is similar to Deadly Shadows: There is a semi-open city hub structured into various districts. Just like in the predecessor, those are divided in loading screens and gradually open up over the course of the game. These can be freely explored until starting a self-contained mission at the designated point. Here lies the main difference to the previous Thief games: instead of sandbox levels, the design is completely modular and can be compared to the Clocktower level in Deadly Shadows. The set-pieces may be relatively open in itself and offer various ways to deal with obstacles or reach the next point of interest, but in the end they result in a choke-point of no return leading to the next area. Movement is also restricted because jumping is only contextual. Rope arrows make their return, but can only be used at a few designated spots. Except for a few minor climbing sequences, the whole game is in first-person.
Garrett can take on guards in direct combat, the system is based on dodging attacks with a designated button and placing blackjack hits at the right moments, but since he is no fighter he usually has no change against multiple enemies. So his main weapon of choice is stealth. It is based on shadows – the light gem in the interface shows how visible Garrett is – and while breaking sight also plays its role, there is no dedicated cover mechanic like in Deus Ex: Human Revolution. However, Garrett can grab onto corners and peek out to avoid running in front of a guard. This means the majority of the time is spent with observing guard patrols, picking a good time to move behind a guard, performing a blackjack takedown and drag the body into darkness before someone sees it. Like in Human Revolution, the takedown is a context based button press instead of a free swing.
In contrast to the previous Thief games there is no loot requirement to beat a mission, but of course there is still plenty of stuff to steal which is directly converted into gold. While the majority of small loot items is just lying around or found by opening drawers, there are also unique items which bring extra money. These are usually hidden and require the usage of a new mechanic: Garrett grabs a painting or book shelf and slowly moves his hand until he feels a hidden switch. By pressing a button, it may open up a secret passage, unveil a safe or disable traps. The lockpicking minigame works similar and requires to find the sweet spot by moving the lockpick around. Safes require the combination which has to manually puzzled out by searching the environment for clues and carefully reading near documents.
Between missions, Garrett can also find much loot in the city by just moving around and keeping his eyes open. However, the most lucrative is to take optional side missions. Here Garrett has to retrieve a certain object – usually it is just a quick burglary within the city, but sometimes they are real self-contained missions. Of course there is a useful usage for money: visiting a shop and stock up on equipment. The obvious choices are depleting resources like Garrett’s trademark arrows which can be used for many tasks, e.g. regular arrows for distracting guards or water arrows to extinguish light sources, food items to heal, or smoke bombs. However, there are also various one-time purchases which reward new equipment to interact with the environment, e.g. a razor to cut out valuable painting or a wrench to open up ventilation shafts, or tool upgrades, e.g. a bigger arrow capacity or more sensitive lockpicks. Also available are items which reward passive bonuses. Additionally Garrett has two magical abilities to his disposal: The swoop move works similar to blink in Dishonored and allows to quickly move forward. Focus (similar to instinct in Hitman: Absolution) slows down time and has several useful perks, e.g. faster lockpicking, easier combat or showing all loot in the area. Those perks have to be unlocked with focus points which are also bought with money.
One of the game’s special features are the difficulty modes. While there are the usual three standard types – the highest does not allow for kills or civilian knockouts – the player can also add several other difficulty options, e.g. disabling focus, more expensive resources, slower movement, no kills or knockouts, no quicksave or permadeath. Because those reward bonus points for bragging rights, the difficulty can only be changed when starting a new game. On the other hand there are also several other interface related options which can be disabled at any time, e.g. loot glow, prompts when approaching an interactive item, enemy alert indicators, objective markers, mini map or ammo counter.
Besides the main campaign, there is also a challenge mode. This mode re-uses some of the game’s mission areas and adds special objectives. The modes are Chain & Gain (there is a timer running out which can only be expanded by collecting loot), Chain & Gain Limited (Chain & Gain with an overall time limit) and Special Loot Hunt (finding as much special loot within a time limit by using the “hot and cold” indicators). The goal is a leaderboards high-score.