Thief is a reboot of the Thief series and throws away most of the lore. 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 of 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 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 they are limited. Garrett has to use his wits and skills to overcome challenges.
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 arrows which can be used for many tasks, or food items to heal.
Garrett has two magical abilities to his disposal: The swoop move works similar to blink and allows to quickly move forward. Focus slows down time and has several useful perks, e.g. faster lockpicking 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 or no quicksave or permadeath. Because those reward bonus points for bragging rights, the difficulty can only be changed when starting a new game.
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 goal is to get a high score on the leaderboards.
The PlayStation 4 version uses the DualShock 4 gamepad's light bar to emphasize the effect of being in the shadow (with a dimmed dark blue) or exposed to the light (with a bright white).