Sigma Theory, is a game where the end goal is to obtain the ultimate technology that will unlock the singularity. You do this by having scientist work on projects. The more scientists you have working, the more work towards this goal gets accomplished. You get scientist by using your agents to Abduct, Seduce, Bribe or Convert them. Each agent comes with a set of abilities and stats. When you unlock a technology you can sometimes apply this to the agents to give them better stats or an ability.
Beginner's Guide to Sigma Theory
When first starting playing you'll only be able to recruit from a limited number of agents. As you play you will unlock other agents to use in future games and/or if you lose an agent. When you lose an agent you will often be granted a randomly selected agent to recruit. You need to the best one of three answers to successfully recruit an agent. If you fail too many times the game will randomly select four agents from the pool. Read the agents profiles, analyze what motivates them and try to pick the best answer that matches that. In future games, if you've already successfully recruited the agent they will be recruited without question.
There's several selections of four agents that will work. You want at least one hacker, one spy catcher, one seducer/negotiator and one elite agent. You only get to pick one elite agent so make a it a good one. I like the woman who is a black belt and is an expert at exfiltration. Black belts usually do the best when extracting scientist from high risk missions and being an expert at getting away is also going to go a long way towards success.
There's a few important things to do early on that will make the rest of the game much easier than it would otherwise be. You want to lower the alert levels, gain intelligence on rival sigma heads and locate scientist to recruit. You'll want to maintain good relations with the other sigma heads but not at the expense of your agents.
Groups will ask you for help and if you betray them you'll probably lose agents so when a rival sigma head asks so don't.
Error on the side of keeping the doomsday clock from advancing because if it reaches zero it's game over. I've never lost by a rival sigma head winning. It's almost always the doomsday clock.
Use your hacker(s) to lower the alert levels. Any agent can do hacking and this uses their int stat. Don't try to hack with agents that have a low intellect. It's better to use high strength agents to recon, infiltrate and counter enemy agents. If you don't yet have a good spy hunter unlocked use a high strength agent to counter instead of capture agents. Elite agents are usually the most successful spy hunters. The elite spy hunter is something you'll need to unlock.
When deciding if you are ready to begin recruiting scientist consider the alert levels and agents available for risky exfilration missions. That is missions where the alert level is 50% or higher. You don't want to use your non-elite seducer agents for high risk missions. They will most likely be killed or captured. This is why having a good hacker to lower alert levels is a big help. Just about any agent will do for exfilration missions when the alert level is zero.
In negotiations with rival sigma heads, getting intel on them is important. Otherwise it's just a guessing game about what to ask. It's also nice to have something to leverage over them like having a surveillance drone there or installing randsom ware. Early-on you want to improve relationships as much as possible. This avoids the possibility of a starting a war and the doomsday counter reaching zero. War causes it to tick down to zero faster.
When choosing what to do with technology, I usually error on the side of whatever moves the doomsday clock back or helps the agents get better at their job. Sometimes if relationships are low it's better to pick that option instead.
Towards the end of the game you'll mainly be recruiting scientist and try to defend against enemy agents. Several countries will be out of the race and their scientists are easily obtained by any agent. Just try to keep that doomdsay clock from reaching zero and try to keep your rivals from getting your scientists. If capturing agents isn't working try countering them. It sends them away for several turns.
Keep in mind this game is in Early Access at the time of writing this little guide. Games evolve over time. I expect the Sigma Theory will be developed to be a much more interesting and challenging game in the future.