3. Story walkthrough
As stated in the introduction, there are three main missable achievements in the game, plus one (Blade in the Crowd) which is to some extent a missable too. After taking a look at them, I'll list the story related (secret) achievements.
Missable achievements
Kill one of your main targets like a true assassin.
Best done on your first real target in Damascus: when you take control of Altair. After some storyline scenes introducing the enemy, make your way to the target, helping your stealth by means of holding A (the "blending"; it slows you down, but you're hardly detectable), in low profile of course. When you're very close to him, use the hidden blade and strike with X.
See 85% of all the memory glitches.
In most the cutscenes, sometimes vanishing chemical formulas will appear on the screen for you to barely see them. The first time you can see one will be in the first scenes of real play after the tutorials, and a note will tell you about these "glitches" (that's how the game calls them), on the left. That's the signal that a secondary point of view is available: to trigger it (the other point of view, I mean), press any button (A, B, ...) quickly after that signal and the camera should change its position. The easiest way to do this is simply spamming A/B/... everytime a dialogues like these triggers. Sometimes buttons can still change the camera (for example in most the dialogues with Al Mualim), continuously as you keep pressing the buttons, but these do not count for the achievement (and are not preceeded by the white formulas), so you can just ignore them. In the following video of the first few seconds of gameplay there's the first example of "glitch" of the game.
Assassinate all your targets with a full DNA bar.
This achievement does NOT require you to have a full health bar when killing your "main target": that's a synchrony gauge, and everyone can confirm health has nothing to do with this achievement.
Some people say you need to complete every investigation/high point/citizens help mission before killing your target - this completes the DNA sequence on your left, the one which appears also in the Animus; each "bar" of the DNA I'm talking about is for an investigation/high point/mission. Another way to understand which DNA I'm speaking about is pausing the game (press Start), then look at the spinning "sticks" just above the Memory Log option (the first of the 6 you can choose): that's the DNA the game means, that's the one I'm talking about. If the stick is white/light blue, you fulfilled that task; if it's blank, you didn't. The Memory Log option itself tells you what the requirements for that stick itself is.
Other people say you need to one-shot every main target with the hidden blade (either low profile or high profile). I tried doing only the second one (abusing of either stealth-assassination, "Blade in the Crowd" style, or the counter with the hidden blade, or assassination with the hidden blade after a taunt of the main target) and that was enough; twice out ot two tries.
Nonetheless, my advice is to do both the things (complete the DNA sequence and insta-kill the main targets with the hidden blade), just to be safe. Moreover, there's an achievement requiring you to climb on every high point; the high points reveal the map, so you should want to clear them sooner or later too. Other achievements require you to clear every citizen mission too. Investigations add extra gameplay and information to the story, so they're worth doing. Do this before killing your targets, and it'll be two birds with one stone. The achievement pops after facing the target of Memory Block 6.
Go through every dialog with Lucy.
Finally, this one. Every time you're pushed out of the Animus, there'll be some scenes after which you're supposed to go in your room to take a rest. You'll be locked in your room right after you enter it, so be careful NOT going inside until you do everything for this achievement. As a matter of fact, after the automatic scenes some dialogues with Lucy and sometimes short events will be available. After all the possible secondary dialogues, you'll be able to interact with Lucy again but she'll start saying repetitive sentences such as "Aren't you tired?" and "Can't talk, gotta work". While these lines are potentially to be repeated forever as you continue to "interact" with her, and do not count for the achievement, you should try to interact until you actually hear them. They're simply the best way to know when you're done with it. Here's the list of the conversations in chronological order:
First time (after Memory Block 1, wait for Lucy to come back): interact with Lucy twice.
Second time (after Memory Block 2): interact with Lucy three times.
Third time (after Memory Block 3): interact with Lucy twice.
After Memory Block 4 there are no secondary dialogues; you can interact with Lucy, just to listen to a "Please, I can't leave until you go to your bedroom" (or anything like that) which doesn't count for the achievement.
Fourth time (after Memory Block 5): interact with Lucy three times.
Fifth and last time (after Memory Block 6): interact with Lucy once and you'll unlock the achievement.
In addition to these conversations, you can read the emails from Lucy's and even the "doc's" computers (not always though). The other few possible scenes consist in examining the closet in front of the bed in your room early in the game (it'll be locked); then examine it again at the end of Memory Block 3 (of course that's after you complete the secondary dialogues outside the room); you can steal the doc's pen drive at the beginning of Memory Block 4, while he'll be staring at the window with a cup of coffee in his hand; and then emails again. Early in the game you should also be able to overhear a conversation between the doctor and Lucy in your room, anyway. They add a bit to the story, but are not required for the achievement.
Story related achievements
Play through the tutorial for the first easy achievement:
Continue playing to unlock this secret achievement. (Secret)
Memory Block 1
One achievement will be earned as you progress:
Continue playing to unlock this secret achievement. (Secret)
Conversationalist - Part I
At the end of Memory Block 1 you'll be outside the Animus: before going anywhere else (especially, DON'T go to your room, that is the only other room you can access), wait for Lucy to come back and interact (triggering a conversation) with her twice. Then you can go to your bedroom, where you'll be locked until the next morning (interact with the bed to sleep).
Memory Block 2
Two achievements will be earned as you progress:
Continue playing to unlock this secret achievement. (Secret)
Continue playing to unlock this secret achievement. (Secret)
Conversationalist - Part II
At the end of Memory Block 2 you'll be outside the Animus again; interact with Lucy three times, then go take a rest.
You should easily manage to unlock Blade in the Crowd along with the second of these two achievements above.
Memory Block 3
Two achievements will be earned as you progress:
Continue playing to unlock this secret achievement. (Secret)
Continue playing to unlock this secret achievement. (Secret)
Note: purely for a slighlty "tastier" approach with the story, it's probably adviseable to make your way to the enemy in Acre first, then in Jerusalem.
Conversationalist - Part III
At the end of Memory Block 3 you'll be outside the Animus again; interact with Lucy twice, then go take a rest.
Memory Block 4
Three achievements will be earned as you progress:
Continue playing to unlock this secret achievement. (Secret)
Continue playing to unlock this secret achievement. (Secret)
Continue playing to unlock this secret achievement. (Secret)
At the end of Memory Block 4 you'll be outside the Animus as usual, but you won't be able to trigger secondary dialogues (except the repetitive ones inviting you to rest) with Lucy.
Memory Block 5
Two achievements will be earned as you progress:
Continue playing to unlock this secret achievement. (Secret)
Continue playing to unlock this secret achievement. (Secret)
Conversationalist - Part IV
At the end of Memory Block 5 you'll be outside the Animus again; interact with Lucy three times.
Memory Block 6
An achievement will be earned as you progress:
Continue playing to unlock this secret achievement. (Secret)
Note: some people have reported a strange glitch which could freeze your game making you fall "in" the ground forever in case you're using two controllers during some parts of this memory block. Plug a single controller and you should be fine.
Note 2: your target is in a place which can be reached from a path just on the right of Acre (there are two side paths on the right as you exit Acre to the Kingdom, in the bottom left area of the Kingdom itself): one of the paths is not available at first, but the other is, and that's the way to the target.
Conversationalist - Final Part
At the end of Memory Block 6 you'll be outside the Animus again; interact with Lucy once to unlock the Conversationalist achievement.
By this point you should have unlocked also the Hungerer of Knowledge achievement.
At this point you should have unlocked Disciple of the Creed (it pops up right as you've finished with the target).
Memory Block 7
Two achievements will be earned as you progress:
Complete Assassin's Creed.
Continue playing to unlock this secret achievement. (Secret)
After the credits, the post-game mode will be available, and that's where you can go after miscellaneous achievements you haven't grabbed on the way.
This walkthrough is the property of TrueAchievements.com. This walkthrough and any content included may not be reproduced without written permission. TrueAchievements.com and its users have no affiliation with any of this game's creators or copyright holders and any trademarks used herein belong to their respective owners.