In season three of "Star Trek: Discovery," Captain Saru makes a somewhat surprising decision and appoints Ensign Sylvia Tilly as his acting first officer. This was a source of considerable debate at the time, particularly in light of the mission to the Verubin Nebula at the end of the season.
Now that quite a bit of time has passed, I wanted to revisit the events of that mission and evaluate the decisions Tilly made while in command of Discovery. I've broken each significant decision or order down according to the following criteria: the circumstances under which the decision was made, what the decision was, and what the general outcome. I've tried to reserve my personal judgment until the end.
Episode: "Su'Kal"
Circumstances: The USS Discovery discovers the source of the Burn - a planet in the Verubin Nebula featuring a massive dilithium deposit, as well as a crashed ship with a Kelpien life sign aboard. The Verubin Nebula is difficult to navigate, and contains deadly radiation and harmful electromagnetic fields.
Command Decision: Captain Saru chooses to lead the away team, leaving Ensign Tilly in command of Discovery.
Outcome: Admiral Vance expresses misgivings, but allows this plan to proceed. He also reveals that the Emerald Chain appears to be attacking Kaminar in order to lure Discovery there to obtain the spore drive.
Circumstances: Discovery jumps into a stable pocket of space within the nebula, which affects the ship's shields - a three-hour repair job, according to Tilly. The away team beams to the planet.
Command Decision: Tilly assumes command and jumps Discovery out of the nebula.
Outcome: Discovery is able to avoid further damage and commence repairs to the shields while the away team is on the surface. Innoculated against the radiation, the away team is able to spend a maximum of four hours on the planet.
Circumstances: While the shields are being repaired, Commander Burnham reports that the away team has found something and growling. Communication with the away team is then lost.
Command Decision: Tilly orders Commander Stamets to prioritize the shield repairs, diverting power from other systems as needed, so they can retrieve the away team.
Outcome: The shield repairs are accelerated. Shortly after this order, an unidentified Federation ship is detected on long-range sensors. Ten minutes out, it does not respond to hails, but sends correct response codes.
Circumstances: With the unidentified ship now two minutes out, the crew discusses how the vessel's presence in the region doesn't really make sense.
Command Decision: Tilly orders Owosekun to scan the area around the ship.
Outcome: The crew discovers that the approaching ship is, in fact, the Viridian, as no one else has the motivation and the means to travel such a distance.
Circumstances: The Viridian arrives at Discovery's location.
Command Decision: Tilly orders red alert. Rejecting the possibility of retreating via transwarp tunnel or spore drive, as it would leave the away team vulnerable, Tilly orders Discovery to cloak.
Outcome: This successfully hides Discovery from the Viridian, though it also deprives them of the ability to use the spore drive. The Viridian cloaks as well.
Circumstances: The crew ascertains that Osyraa must be able to track Discovery's jumps, and that she must require the ship, as she didn't open fire immediately upon arrival. Stamets reports that Discovery cannot jump for another 30 minutes. Osyraa hails Discovery.
Command Decision: Tilly orders Stamets to find a way to repair the shields in the next ten minutes. On a comm channel, Osyraa claims to want Discovery and her crew for "leverage."
Outcome: Repairs to the shields continue to be prioritized as the situation escalates.
Circumstances: Unbeknownst to either ship, Su'Kal creates a spatial disturbance, destabilizing Discovery's dilithium. Discovery's engineering team is able to contain the effects, but both Discovery and Viridian lose their cloaks.
Command Decision: Tilly orders weapons ready.
Outcome: The situation with the Viridian continues to escalate.
Circumstances: Discovery's shields have been repaired to 54% as the standoff with the Viridian continues.
Command Decision: Tilly orders Stamets to prepare to jump Discovery to safety, rather than allow it to fall into Osyraa's hands. Booker volunteers to remain behind in his own ship to retrieve the away team.
Outcome: Over the protests of Stamets, this plan is put into action.
Circumstances: Osyraa hails Discovery again. Osyraa notes that Discovery has not yet jumped away, and deduces that Captain Saru must be in the nebula. She also claims that the structural weakness in Viridian that Discovery exploited in "The Sanctuary" has been repaired. Booker leaves the shuttlebay in his ship.
Command Decision: Tilly orders the spore jump.
Outcome: Emerald Chain soldiers beam into the engineering lab and attack Stamets before he is able to follow this order. Viridian ensnares Discovery with some kind of tendrils, and Emerald Chain soldiers begin capture the ship. Ultimately, they are able to take the bridge and remove Tilly from command, jumping away just as Booker and Burnham emerge from the nebula in Booker's ship.
Especially since the Discovery is a comparatively ancient ship with newer hardware just glued on.
Much of its onboard design and technologies (like the hull materials/structure, and core computer systems) are similar to what the ship was originally fitted with, which would be millennium-old designs at that point in time.
A newer starship of the time might have fared better, but Discovery did not have that advantage.