Black Flag is the 4th Assassin's Creed game. Desmond Miles, the modern day protagonist in the Assassin's Creed series has died. Abstergo Industries have taken his memories and are continuing to explore them in their machines known as the Animus. In the modern day, you are playing as an unnamed character and are exploring part of Desmond's memories, so play as a pirate called Edward Kenway.
Early on, Edward kills an Assassin, and assumes his persona in order to pursue profit. It turns out the Assassin he killed had gone rogue and was working with the Templars to find a structure known as the Observatory. Edward sees his chance for riches and decides to pursue this with his band of pirates.
Although not a trained Assassin, Edward behaves exactly like one. So you still have the normal movement of scaling buildings, sneaking around, performing assassinations with hidden blades or his other equipment. Edward is equipped with two swords instead of one, can use revolvers, poison darts and smoke bombs.
The game world is comprised of many islands, some have cities, whereas other islands just have a treasure chest or similar; much like the world in Legend of Zelda: Wind Waker. You will still be doing the usual Assassin's Creed gameplay, climbing massive viewpoints, finding chests, assassinating targets, with new ideas such as hunting to craft weapons and upgrades.
The buildings in this game are generally small though, and you will be doing a lot of stealth utilising bushes. I found the climbing quite awkward in this game where I was accidentally climbing, accidentally leaping off buildings, or seem to get stuck when I felt Edward could easily pull himself up.
Early on you acquire a ship known as the Jackdaw which is used to travel the seas. There's many battles to be had at sea, giving you rewards once you defeat your enemies. There's side cannons to fire horizontally, mortars for heavy ranged attacks, and explosive barrels can be dropped directly behind. You can also upgrade ramming strength of your ship. After destroying each enemy ship, players can either sink the ship and gain half of the loot, or board it to take the lot. As you board, you then fight the crew and are task with defeating a certain number of them in order to claim victory. You can then use their ship to repair your ship, add the ship to your fleet, or bribe officers to reduce your notoriety.
Your fleet can be sent on trading missions via your ship's quarters and is another way of accumulating wealth. It's a simple, shallow mini-game, so soon becomes tedious.
There's plenty of distractions at sea. There's plenty of ships to attack/avoid as you see fit, plenty of islands with treasure, strongholds to conquer, sea creatures to hunt, and sunken treasure to find. The strongholds can be attacked with your cannons, then you must battle on foot much like conquering a ship. The sea creatures like sharks and whales must be repeatedly harpooned to the death (this is visually brutal). To find sunken treasure, you can swim underwater, avoiding sharks and maintaining air supply via barrels. The swimming controls feel awkward, and you cannot attack, so must use the seabed as cover to hide.
The game has a lot of gameplay carried over from Assassin's Creed 2 (I didn't play the third one, so it's probably similar to that too). You can hire prostitutes to have a moving crowd to hide in, or hire a team of mercenaries to assist you when you fight. This is a very underused feature and you will forget that it exists.
The biggest gripe of the game is that you are very overpowered in combat, meaning you can easily get yourself out of situations where you have failed using stealth. The guards are usually very dumb so give up quickly after spotting you, are easily baited by whistling, and seem to stand in danger if they notice their dead colleagues. When things do go wrong, enemies often attack you one-on-one despite surrounding you with several men. A prompt appears to counter-attack then you can disarm, throw, or go for the kill. It's very shallow combat that has never been a strong point of the series.
When you are back in the real world, you walk around the Abstergo offices in first-person. A mysterious guy contacts you and ends up giving you missions to steal data and send it on to him. These usually have simple but varied puzzles involved in them.
I always think the games should be about half the length but twice as difficult. It's a game series that implements loads of ideas okay, rather than doing a few things really well. The game does start to feel boring before the half way point, and that's if you try not to get distracted by all the side quests.