A series of 4 episodes (well, an intro and 3 episodes), this was part of last weeks new expansion, Agents of Yesterday where you could start a character in the 23rd century TOS timeline.
I didn`t do that personally, nor am I intending to, but I did play the story which Daniels (from Enterprise) gives you, and which leads up to two new stories in the Future Proof chain.
In the first episode, you are transported back to the mid 23rd century, where the Romulans are about to make a pact with the Nak`Hul. They will provide the Romulans with a Doomsday Machine, and you, disguised as Tal Shiar agents, must discover what is going on.
However, you are soon discovered as a mysterious Envoy arrives at the scene, and the battle moves into orbit to destory the activated machine.
You manage this, but at a great cost as the colony world is destroyed when the Doomsday Machine crashed into from orbit.
You then move on and have to save the scientist Kal Dano, who will in turn become a Temporal Agent himself (again, see the Future Proof storyline), before being killed and found by Captain Archer in the 22nd century.
However, you fail in doing this as Voth interfere, and you go back to Archer`s era to prevent the capture of the Tox Uthat, which would otherwise fall in their hands as Picard wouldn`t have found it. You learn though that the one he destroyed was a fake and it rests in Starfleet`s vault, which you breach in the middle of the Dominion attack on Earth during the Dominion War, stopping the Voth and keeping the timeline steady.
In the final episode, you travel to the Delphic expanse... in another reality. This Kelvin Timeline is the line created in the reboot movies by JJ Abrams, and you fight alongside the android crewman from that movie franchise to stop the Sphere Builders.
Under guidance from the Envoy, they have enlisted the Klingons in this alternate reality to do the job the Xindi did in the regular one, and using sabotage techniques you manage to destroy the Sphere.
While Daniels gets wracked more and more due to injuries "elsewhen", you await further orders to see how this plays out... to be continued in the new episodes of Future Proof.