My guides tend to be straight forward. I will provide solutions to every mission in the game and not just hints.
If you don't want to rob yourself of the fun you could have with this game, move along.

There are spoiler-free guides out there and I strongly recommend using them to try and solve the game by yourself before you use my guide.

You can also find help in steam forums or on the discord support channel https://discord.gg/arg

Please enter the puzzle piece codes I provide before entering the solution to the puzzle if you aim for all obtainable achievements.

Accuracy
Description: Complete an investigation on your first try
-> If you use my guide you should get this achievement in the first mission.

Endurance
Description: Submit over 15 responses in one investigation
-> Just enter 15 wrong solutions in a mission

The Agent Map
Description: Unlock The Agent Map
The Situation Room
Description: Unlock The Situation Room
The Cybertech
Description: Unlock the Cybertech class
The Scientist
Description: Unlock the Scientist class
The Spy
Description: Unlock the Spy class
The Soldier
Description: Unlock the Soldier class
---> Those should automatically unlock while playing

Achivements for completing missions:
New Recruit
Description: Complete "Mission 3"
Chemistry
Description: Complete "Mission 6"
Medical
Description: Complete "Mission 11"
Bones
Description: Complete "Mission 15"
Hunter
Description: Complete "Mission 19"
Occult
Description: Complete "Mission 25"
Analyst
Description: Season 2: Complete "Mission 4"
Local Enforcement
Description: Season 2: Complete "Mission 10"
Storm Chaser
Description: Season 2: Complete "Mission 14"
Body Trail
Description: Season 2: Complete "Mission 18"
Die Hard
Description: Season 2: Complete "Mission 23"
Whitechapel
Description: Complete the "Whitechapel" Special Ops
Denial of Service
Description: Alone in the Dark Web: Complete "Mission 1"
Facility Inspector
Description: Mother Russia: Complete "Mission 1"
GEOINT Specialist
Description: Mother Russia: Complete "Mission 3"
Too Big to Fail
Description: Season 2: Complete "Mission 27"
Msika Archivist
Description: Mother Russia: Complete "Mission 5"
Black Site
Description: Season 2: Complete "Mission 30"
Primum non nocere
Description: Alone in the Dark Web: Complete "Mission 4"
Conclusion and Foresight
Description: Alone in the Dark Web: Complete "Mission 8"
Sleeper Agent
Description: Completed Ahnayro Sandbox Campaign
Pathfinder
Description: Complete the secret missions


Broken achievements:
Medic
Description: Joined at least 5 calls for senior agents
On Duty 1-10
Description: Complete Live Event 1-10
Viper Venom
Description: Completed COM-Division Event 1
Russian Connection
Description: Mother Russia: Complete Live Event
Halloween 2015
Description: Complete Halloween 2015 Live Event
Collaborator
Description: Joined at least 5 calls for collaborators
Shock Trooper
Description: You pre-ordered the game
Futurology
Description: You backed the project on Kickstarter
Greenlight Operative
Description: You purchased the Steam funders package on Kickstarter

Mission 1: Basic training
Hidden Meaning
Puzzle piece code: Thallium
Solution: Target located
Look Closer
Solution: Successful extraction

Mission 2: Training ground
Voice in the ether
Puzzle piece code: Cadmium
Solution: In position
Orders from HQ
Solution: INTERCEPT TARGET IN SHANGHAI

Mission 3: Advanced Preparation
Merchant missing:
Puzzle piece code: Indium
Solution: Hermetic Order of the Golden Dawn

Mission 4: Resupply
Paper Trail
Puzzle piece code: 87.62
Solution: Morpho Medical
Stock up
Solution: 200276

Mission 5: Radiation
Corrupted File #1
Puzzle piece code: 101.17
Solution: DN98A1
Corrupted File #2
Solution: 3053
Lost File #1
Solution: Iodine

Mission 6: Clinical Trials
New staff
Puzzle piece code: 180.94788
Solution: Holmesburg Prison
Current activity
Solution: 665-XX8
Bugged
Solution: ASD8F78

Mission 7: Wire trap
The conversation
Puzzle piece code: 3306
Solution: Bone Marrow
Improved methods
Solution: Liquid

Mission 8: Substance
Dr.Myrick
Puzzle piece code: 3695
Solution: M55

Mission 9: Social Engineering
Ties that bind
Puzzle piece code: 1115
Solution: Superterram Life Sciences
Sheep's Clothing
Solution: RBG55432443

Mission 10: Supplier investigation
Supply inspection
Puzzle piece code: 505.08
Solution: 108.2
Full compromise
Solution: Hexmo Plugin
Dark truths
Solution: Sunny Forest Orphanage

Mission 11: Raid on Rosenberg
Past mistakes
Puzzle piece code: 115.78
Solution: 3409
The raid
Solution: DAK-29

Mission 12: Test results
Irregular levels
Puzzle piece code: Winkler
Solution: Felis Silvestris

Mission 13: Two headed eagle
The letter
Puzzle piece code: Oganessian
Solution: Brother Metemius
Brother Metemius
Solution: Ancient Brotherhood of the Solar Temple

Mission 14: The ancient brotherhood
Past posts
Puzzle piece code: Marguerite
Solution: Hellfire Club

Mission 15: Hellfire
Sacred ground
Puzzle piece code: Hofmann
Solution: John Wilkes
Lost souls
Solution: Ann
Voice from beyond
Solution: December 21st 1760

Mission 16: The rare and the exotic
Location is everything
Puzzle piece code: 19.84
Solution: Lock & Key Storage
Contact information
Solution: 224-678-0415

Mission 17: Insider trading
Playing the market
Puzzle piece code: 15.10
Solution: Magic the Gathering
A perfect card
Solution: Cartamundi

Mission 18: The manor house
Element of surprise
Puzzle piece code: 18.95
Solution: 03:12:45
Blast radius
Solution: 310

Mission 19: Stronghold alpha
Support
Puzzle piece code: 12.02
Solution:
Go to the global map and select "Mission 19 Stronghold Raid".
Click on "Front door" and enter "A0113" as the code.
Click on "Room Alpha" and enter "2837" as the code.
Click on "Room Charlie" and enter "2258" as the code.
Click on "Room Echo" and enter "2737" as the code.
Now go back to your mission and enter "918TC" to finish this mission.

Mission 20: Lasting effects
Unstable growth
Puzzle piece code: 27.2
Solution: Thymus
Adverse reactions
Solution: Chrysopoeia

Mission 21: Aftermath
Forensic investigation
Puzzle piece code: 13.81
Solution: Okinawa
Ground levels
Solution: Kadena Air Base

Mission 22: Nu orb
Material analysis
Puzzle piece code: 33.29
Solution: Granite
Abyssal signature
Solution: Cleopatra's needle

Mission 23: Countermeasures
Containment
Puzzle piece code: 17.49
Solution: Oriental institute
The persian galery
Solution: A10
Inscription
Solution: Ahura Mazda

Mission 24: Ambush
Homebrew
Puzzle piece code: 2.27
Solution: SERF
Cloak and dagger
Solution: negative index metamaterial
One shot
Solution: Adelphi

Mission 25: The abyss
Into the abyss
Puzzle piece code: 19.79
Go to the global map and click on "Mission 25 Rift Portal".

Click on "First entity" and enter Stolas as your solution.
Click on "Stolas" and enter Beta aquarii as your solution.
Click on "Local authorities" and enter hsbc as your solution.

Click on "Second entity" and enter amdusias as your solution.
Click on "Amduscias" and enter Berlioz as your solution.

Click on "Third entity" and enter Focalor as your solution.
Click on "Focalor" and enter Montevideo Maru as your solution.
Click on "Emergency Call" and enter Shamrock as your solution.
Click on "Abyssal writing" and enter yhwhygvfvcoe as your solution.

Now go back to your mission and enter "CTK5B" to finish this mission.

Mission 1: Research training
Image retrieval
Puzzle piece code: Giotto
Solution: Rider-Waite Tarot Deck
Tracking interest
Solution: Uruguay
Key links
Solution: The fellowship of the rosy cross

Mission 2: Mapping Patterns
Former glory
Puzzle piece code: Phidias
Solution: Le trianon akasaka
Wind warning
Solution: 26.3
Bloodshed
Solution: Cortes

Mission 3: Technical Training
Lookup
Puzzle piece code: Tolstoj
Solution: Internet assigned numbers authority
Technical origins
Solution: Hong Kong

Mission 4: File inspection
Ghost data
Puzzle piece code: Beethoven
Solution: Rift
Second look
Solution: Void
Image decryption
Solution: UTAH Data Center

Mission 5: Anomaly in the desert
Missing link
Puzzle piece code: Erinome
Solution: Brandon Rooney
Cause and effect
Solution: Dust storm

Mission 6: Lost in time
Last seen
Puzzle piece code: Metis
Solution: 992 HPA
Strange origins
Solution: Ashes

Mission 7: Local death
Final shot
Puzzle piece code: Io
Solution: Elder Sign
Last breaths
Solution: Quachil Uttaus
Seared flesh
Solution: symbol of chaos

Mission 8: Under the sand
Local help
Puzzle piece code: JIMO
Solution: marree man
Buried warning
Solution: Dieri

Mission 9: Seek and destroy
Useful functions
Puzzle piece code: Dia
Solution: toas
Area of effect
Solution: Kanyaka Station
Into the darkness
Solution: A4589W

Mission 10: The liar
Dark signal
Puzzle piece code: Leda
Solution: Arecibo Message
Infestation
Solution: Labidura Herculeana
Old scraps
Solution: Charles Carroll Taylor

Mission 11: Post-Mortem
Opal5
Solution: 19430122
Covert investigation
Puzzle piece code: Le Verrier
Solution: Malmstrom Air Force Base
Wingspan
Solution: Overland

Mission 12: Lorenz Weather Mapping Group
Cold Case
Puzzle piece code: Berlin Observatory
Solution: cherufe
Generous donor
Solution: 381768
Background check
Solution: NRO

Mission 13: Cluster analysis
Sysconfig
Puzzle piece code: Galle
Solution: EXO56
Predicting the impossible
Solution: Caracas

Mission 14: Power and influence
Inside job
Puzzle piece code: Voyager 2
Solution: caracas.diplo.de
Future forecast
Solution: Parque tecnologico sartenejas
Watering hole
Solution: bar y restaurante el meson de cervantes
Bird
Solution: cartel de los soles
Ball
Solution: Base Aérea General José Antonio Páez

Mission 15: Transmission
Map
Puzzle piece code: Jeffreys
Solution: iglesiasanmartin
Coat of arms
Solution: Martin Schwegelin

Mission 16: Dial tone
Call for help
Puzzle piece code: Keeler
Solution: Fairfield Inn
Prototype
Solution: outflow undercurrent interaction and jetstream adjustment

Mission 17: Lockdown
Safe house
Puzzle piece code: 1610
Solution: National severe storms lab
Tamper proof
Solution: 315eff670cfe690798ffb55725e0c860d6ac74f911bd0619f0f85c7561a3e51f1fe7a6c33772bf451d2b4558b5f51c4d67b64bfc4cc46c78f3c9f7b849d359a6
Decoy message
Solution: Lustre
Dead end
Solution: G07

Mission 18: Forcible entry
Protocol
Puzzle piece code: Roche
Solution: Wiegand
Data dump
Solution: 54
Intrusion
Solution: ESPKey

Mission 19: Leverage
Trail
Puzzle piece code: Sycorax
Solution: 344547642
Dirty secrets
Solution: snuffvideocentral.onion

Mission 20: Airport pickup
Cancellation policy
Puzzle piece code: Margaret
Solution: Grandhotel hessischer hof
Special treatment
Solution: A13
Excuses
Solution: KFW Ipex-Bank

Mission 21: Preemptive strike
Canary
Puzzle piece code: portia
Solution: humska
Severed lines
Solution: Picot-1
Silent sky
Solution: PAS-8

Mission 22: Way of the knife
Advanced persistent threat
Puzzle piece code: Cressida
Solution: 114724
Mockingbird
Solution: korea ryonbong general corporation
Dangerous substance
Solution: abrin

Mission 23: Cleanup job
Sefety meassures
Puzzle piece code: Mab
Solution: glass re-locker
Numbers game
Solution: 1857
Out of sight
Solution: Lotus

Mission 24: Gathering storm
Rental car
Puzzle piece code: API
Solution: BTR-80
Echoes in the wind
Solution: Shenlong
Falcon nest
Solution: Centro Financiero Confinanzas
Control panel
Solution: Teramobile laser

Mission 25: Authentic Infection
Malware sample
Puzzle piece code: chloris
Solution: zeus
Encrypted traffic
Solution: Setdrop 27.34.142.51

Mission 26: Market crash
Suspicious trading
Puzzle piece code: Theia
Solution: High Frequency trading
Deep end
Solution: Instinet
Source code
Solution: Smart order router

Mission 27: Too big to fail
Material design
Puzzle piece code: Ushas
Solution: Polycrystalline Silicon
Contamination
Solution: silicon tetrachloride
Manufacturing process
Solution: Vortex Rite

Mission 28: Covert compromise
Broken paths
Puzzle piece code: sojourner
Solution: National Security Bureau
Access granted
Solution: ennead

Mission 29: Eyes shut
Primary infiltration
Puzzle piece code: nozomi
Solution: HILSHIRE,CROFT,HASKIN

Mission 30: Lab core
Breakthrough
Puzzle piece code: MAVEN
Solution: Vanessa Baup

The Msika files

Mission 1: A russian connection
A very cold case
Solution: State research center for applied microbiology
Without a cure
Solution: project factor
Missing links
Solution: Vector Institute

Mission 2: The sprawl
Bio hazard
Solution: Marburg Virus
The strain
Solution: Variant U
Occult vectors
Solution: veles
Worst case scenario
Solution: touch piece

Mission 3: Infection path
Targeted attack
Solution: Istanbul
Ideal placements
Solution: port kavkaz

Mission 4: ВРИ, ДА ПОМНИ
Under construction
Solution: SGM Group
Hot of the presses
Solution: 2011-0418

Mission 5: Lost Carcosa
Suit up
Solution: Chemturion
Shipment received
Solution: irtysh
Broken codes
Solution: krasnodar international airport
Port authority
Solution: D15

The crimson library

Mission 1
Guardian
Solution: Bathin
Replication signature
Solution: 380244
Server location
Solution: Nagasaki

Mission 2
Guardian
Solution: Gaap
Replication signature
Solution: 848311
Server location
Solution: Beijing

Mission 3
Guardian
Solution: Morax
Replication signature
Solution: 162029
Server location
Solution: chippewa falls

Mission 4
Guardian
Solution: Zepar
Replication signature
Solution: 541680
Server location
Solution: Kiev

Mission 5
Guardian
Solution: Gusion
Replication signature
Solution: 881625
Server location
Solution: Houston

Mission 6
Guardian
Solution: Vual
Replication signature
Solution: 279115
Server location
Solution: Athens

Mission 7
Guardian
Solution: Beleth
Replication signature
Solution: 162808
Server location
Solution: Goderich

Mission 8
Guardian
Solution: Dantalion
Replication signature
Solution: 589369
Server location
Solution: Wake up

Mayday

Mission 1: Missing Agent
Overboard
Solution: Glens Falls
Last Words
Solution: Cooper's Cave
Unsafe
Solution: 439543
Well documented
Solution: Obicor

Mission 2: Adverse effects
Paper trail
Solution: 7fff80
Living proof
Solution: Combativeness
Momentum
Solution: Zero Gravity

Mission 3: Whitechapel
Litterature review
Solution: Elanor Lane
Slipping away
Solution: 13 Depot Square
Trapped
Solution: vulpes

Mission 1: Four pillars
Real life
Solution: Prince Ikey C
Wheels of steel
Solution: Regina Saddler
Uprock
Solution: Lein Figueroa
Word is bond
Solution: Franklin Armory Men's Shelter
Wrong side of the tracks
Solution: Lee Quiñones

Mission 1: Privatization
Blame
Solution: 12 August 2000
Funding Cuts
Solution: S.2845

Missions 2: Research History
Experiment 42-K
Solution: Sun Streak
Facility 389
Solution: Chemical Corps
Experiment 7639-C
Solution: Psychophysical Research Laboratories

Mission 3: Conflict in Europe
Killed in action
Solution: Carmine Pecorelli
Covert payments
Solution: Florio Fiorini

Mission 4: The Phoenix Facility
Valuable Substance
Solution: Dimethyl sulfoxide
Suspicious Facility
Solution: Cryo-Care Equipment Corporation

Mission 5: Origin Story
Luxury Suite
Solution: King and Prince Hotel
VIP
Solution: Colin Gubbins



Mission 30(?): Site Theta Core
Guardians
Solution: Pulsa d'Nura

Chemical Warfare
Solution: Physostigmine

Core
Solution: 159311354

Mission 1: Tired eyes
Open and shut
Solution: Autosuggestion

Breadcrumbs
Solution: Posthypnotic amnesia

Memento Mori
Solution: Sengai Gibon

Mission 2: Sins of the father
Good intentions
Solution: Medical Research Volunteer Program

Orders from above
Solution: Van Murray Sim

Life work
Solution: Temazepam

Mission 3: Enhanced interrogation
Headhunting
Solution: Behavioral Science Consultation Team

Best practices
Solution: Mitchell Jessen and Associates

Broken mind
Solution: Aberrant

Mission 1: Operation Violet Sunrise
Primer
Solution: Cloak

Propellant
Solution: Kunduz

Casing
Solution: Pushtu

Chamber
Solution: BDB

Contact
Solution: DACB

Mission 2: Ashes to ashes
The report
Solution: Astoria

The agent
Solution: 40

The incantation
Solution: Vade Retro

Mission 3: Operation hidden crescendo
I heard there was a secret
Solution: Alto

Eye of the turtle
Solution: correct horse battery staple

Blinded with science
Solution: Infrasound

Find your way
Solution: Beethovenhalle Bonn Germany

Paint it black
Solution: Eurocopter EC635

Bridge over trebled water
Solution: South bridge

Fire of the valkyries
Solution: Fire

Mission 4: Deadbolt
Jane Woe
Solution:

Artefacto
Solution: Hand of Glory

Mea Culpa
Solution: Tom Ketchum

4-2-2-8-3 answer pattern, [email protected]; sister: Vera Petrovna; hinduism; protector (master of wisdom)

Mission 1: NREM
Mark of the beast
Solution: Herman Webster Mudgett

Mission 2: NREM 2
The blind
Solution: Albert Jackson Tirrell

Mission 3: NREM 3
Forbidden doors
Solution: Howard Carter

Mission 4: REM
Lost lands
Solution: Nyarlathotep

Mission 1: Oakcottage
An old letter
Solution: Ulthar

Mission 2: Baby's day out
First words
Solution: 34 East

Second Wave
Solution: Penn Museum

Third Time the Charm
Solution: Mummy

Mission 4: Symphonica Allegro
Leichte Klaviermusik
Solution:

The images for this last part are missing in game but a friend of mine managed to find them.
Here they are, in case anyone wants to give it a shot:

Mission 1: Medicine
Does it hurt when I do that?
Solution: Vampire Organ
Y'all m**** need jesus
Solution: Banishment

Mission 2: Cheap Organ, Free Death
Tracks in the red dust
Solution: Cape Town
Tag the target
Solution: Biocare institute

Mission 3: A different kind of jungle
Digging the red earth
Solution: HOGUZK287

Mission 4: Dealing with darkness
Bio (don't) care
Solution: HD83KS0

Mission 1: Clash in the dust
Snakes, it had to be snakes
Solution: SH-588

Go to hell
Solution: HG-947 WA-467

Fangtastic
Solution: UG-749 FI-146