‘Scavengers’: Go rogue or go home
Booker’s vessel — minus Book, but not his cat — appears at Starfleet Headquarters. A recording left by Book details his discovery of a "black box" from a starship destroyed in the Burn, found in the slave labor regions run by the Orion/Andorian syndicate, where Book has gone missing and may now be held captive. Such a black box could be a key piece of evidence in understanding how, and perhaps from where, the Burn originated. This is a mystery very much on Burnham’s mind, and she wants to undertake a rescue mission to find her missing friend as well as this black box of possible evidence.
The catch: Admiral Vance needs Discovery on standby for something more urgent to the greater good of Starfleet, so Saru can’t sanction a mission for one man (who, by the way, isn’t even a member of Discovery‘s crew), even if it might shed light on the Burn. So Burnham decides to go rogue and disobey a direct order to rescue Book. She recruits Georgiou ("You had me at unsanctioned mission") to help her.