- In a nutshell, what happens and who are all these characters that appear? - Is fireteam crimson meant to be the players?
- Nothing that really matters.
- Yeah, that’s the player team.
I did play Spartan Ops and I have the same questions
The Infinity returns to Requiem after the events of Halo 4 in an effort to study whats going on with the Forerunner installation that once housed the Didact. It’s now supporting a Covenant cell led by Jul M’dama and so Spartan teams are required to help defend the UNSC in its studies. Fireteam Majestic is one of those teams and they are the story characters to Fireteam Crimson’s gameplay. Majestic is largely in service to the plot whereas Crimson is the gameplay element representing the player are fighting alongside them in the area or are doing things in service to the Majestic plotline.
Doctor Halsey is also aboard the Infinity who is there to help study Requiem in absence of anyone else more qualified if there ever was any. Her efforts to understand the Forerunner’s and her discovery that John-117 was here as well as a growing paranoia of the UNSC leads her to communicate with Jul M’dama when he asks for help regarding the Librarian. She’s abducted and now forced to help the Covenant. The UNSC are forced to assassinate her as shes that high-value and Majestic is sent to stop Spartan Palmer from killing her on orders from Captain Lasky. Ultimately Halsey survives and loses an arm in the process and escapes with Jul M’dama from Requiem.
Spartan Ops was meant to be longer and expand on things but that didn’t happen and so the fallout of Halsey joining the Covenant wasn’t addressed. As such she later escapes Jul with aid of Fireteam Osiris at the beginning of Halo 5. Halo 5 didn’t expand upon her reasoning and thus it’s a dropped plot-point in favor of A.I gone rogue.