The Big Synopsis
Spoilers through "Turns of Events"
In 2382, the U.S.S. Excelsior returned to service after two years in mothballs. Her captain is Rachel Cortez, a half-Vulcan (but mostly-human) young captain; the X.O. is Lieutenant Commander Alcar Dovan, a middle-aged Bolian who has recently returned to service after a post-war leave of absence.
The ship encountered numerous technical and personnel difficulties at Starbase, from the antics of one Crewman Kinash Adow (an arrogant engineer) to the discovery of a stowaway named Simon Westlake (a young man whose Elarin's Syndrome handicaps his mind and will eventually threaten his life) to Dovan's conversation with the shockingly well-informed and protocol-ignoring Nuncian science officer, a petty officer named P'chk'ro'ta. Eventually, the ship was dispatched to Valandria, a Delta Quadrant civilization near the Alpha Gateway that recently acheived warp drive, and whose warrior-matriarchal culture has ruined two prior attempts at first contact. Officially, the mission Captain Cortez is given is to open diplomatic relations with the Valandrin. Unofficially, Captain Cortez was in contact with a mysterious figure called "General Brahms," who called the Valandria mission a "cover story" for "the Anbar." A "Lieutenant Yubari" was said to be assisting in this second mission, but without full knowledge of the operation.
Upon arriving at Valandria, the Excelsior is promptly attacked as part of a formal greeting challenge. With Lt. Alex Rol at Tactical, the Excelsior aquits itself admirably, and Captain Cortez is invited to bring a small group to meet with the Valandrian leadership: Premier Betra-Na, General Sorid-Gee, and Matriarch of Justice Astrin-Sa. This meeting goes south fairly quickly, with Sorid-Gee accusing the Excelsior team of being the fulfillment of prophecy, which speaks of the destruction of the Valandrian race. At the same time, Captain Cortez is becoming increasingly suspicious of the advanced technology the Valandrians display; from their huge space station Vigilance to their powerful orbital defense network, Valandrian engineering is well ahead of where it should be. In short order, Betra-Na agrees to take the Excelsior crew to the source of their prophecies: an entity she refers to as "the Oracle." Meanwhile, Sorid-Gee, who believes Betra-Na is leading Valandria to its destruction, orders her military forces to make "preparations."
Cortez's team is beamed down to a mountainside area on Valandria where they quickly learn that none of their equipment works, due to some kind of ambient energy-dampening radiation.
At the very moment the Excelsior team beams down, the Excelsior itself, commanded by Alcar Dovan, is attacked by the Valandrian defense network and simultaneously boarded by an armed team of Valandrians. Having lost his sensor lock on and communications with the Away Team, he is forced to withdraw.
On the plnet, oblivious to this, Betra-Na leads the team to the Oracle's cave, explaining on the way that the Oracle is responsible for all technological development on Valandria in the last six hundred forty years.
A man steps out from the shadows and says, "Ah. It's about time the Federation got here." He is wearing a blue Starfleet uniform. At Betra-Na's prompting, he prophesies about the "aliens", and explains that, on April 4th, 2402, the Federation Fleet Yards at Valandria were destroyed at a cost of 87 Starfleet vessels. The loss of Valandria, he explains, "is widely regarded to have been the turning point in the War." He is "unable to access further information; database corruption is extensive."
Chief Engineer Lorhrok points to the wall, and Cortez reads from a bulkhead melded with the rock, "NCV-83124... U.S.S. Oracle."
The Oracle reports "holomatrix damage." Captain Cortez's life suddenly gets a whole lot more interesting.
On the Excelsior, the Valandrian assault team has been contained. While waiting for Crewman Adow to modify a shuttle for stealth, Dovan, Dr. Sharp, and Special Ops chief Amara (a conveniently telepathic redshirt) interrogate the prisoners. They learn very little except that Sorid-Gee's older sister was once Premier, but was slain by Betra-Na shortly after Starfleet's last attempt at first contact--and over the same disagreement: does prophecy show that the Federation is the bringer of their destruction or their salvation? From this, Dovan draws the conclusion: perhaps they can use the Betra-Na/Sorid-Gee rivalry to their advantage.
Of course, he doesn't know that that's unnecessary. On the planet, Captain Cortez converses with the Oracle, the vessel AI of the U.S.S. Oracle. He has been trapped in this mountainside since the Oracle was downed at the Battle of Valandria in 2402. Crashing through time and space, it impacted here. The Oracle makes vague references to "Theyven" and "the Many" as the culprits of much of the cultural contamination here, and describes his own technological assistance to the Valandrin as a corrective, rather than the flagrant disregard of the Prime Directive it appears to be. Unfortunately, in its badly damaged state, little more sense can be made of it. This is okay, because, at just about this time, Sorid-Gee interrupts the affair by attacking the entire party with shock troops. Since energy weapons do not function, the battle is fought with swords, large heavy objects, and fists.
Aboard the Excelsior, Amara is preparing to launch a rescue mission when he suddenly seizes up, screams, and collapses. Doctor Sharp describes the symptoms as an inflammation of the brain, but otherwise inexplicable. Dovan takes command of the away mission. Eventually locating the away team, they take the shuttle into the atmosphere, alerting the Valandrian military to their presence. The rescue itself is disspiriting: although Betra-Na and Cortez's party was victorious, it came at a high cost. Such named redshirts as Marine Captain Hastings and Diplomat Rick Hunter are dead. Chief Engineer Lohrok, a member of the main cast, is dazed, and, worst of all, Captain Cortez is wounded and in critical condition.
At Lorhrok's insistence, Dovan and he retrieve the Oracle from the cave and escape the planet, but only after a harrowing air-to-air dogfight. Back aboard the Excelsior, the disease that infected Amara has been spreading, but, still, the cause cannot be identified. Some people who are in contact with the infected catch it immediately; others do not. Some who are in entirely different parts of the ship are getting it, too. All show the same symptoms: instant onset of extreme pain and unconsciousness. At this point, over a dozen have contracted the inflammation, and the rate of spread is such that the Excelsior would never reach Starbase 900 before the entire crew succumbed. In a moment of lucidity, Captain Cortez orders Dovan to "Get these out of here... alive." Dovan promises to do so, and, as Cortez enters an induced coma, he takes command of the ship.
Concluding that this infection can only be a biological attack of some sort, Dovan declares that they only have one option. "Mr. Lorhrok, ready your engineering teams.
"We’re going back in."