The core of this episode is a bit weak and cliched but it is surrounded with decent character work.
Book’s ship turns up at Star Fleet HQ with only Grudge the cat on board. With Saru keen to prove Discovery’s reliability to Star Fleet he forbids Michael from going on an impromptu rescue mission. Naturally Michael goes on an impromptu rescue mission.
The mission itself involves Michael and Georgiou visiting a junkyard planet that’s just some sort of rusty mineral processing factory but filmed with a yellow filter to look alien. The factory is run by the bad-guy organised crime gang and populated with slave-prisoners. The slaves can’t escape because they have devices on their necks that blow their heads off if they cross the perimeter. Book is one of the prisoners etc. It’s fine but you’ve seen it all before.
What is better here is that episode finally makes an attempt to deal with the question of Michael’s propensity for rogue missions in a way that is neither court-martial-life-imprisonment nor a pat-on-the-head and praise for what a cheeky scamp she is. The set-up for her disobedience is (more or less) a bit of a genuine dilemma for both Michael and Saru and the consequences for her insubordination are significant and have impact but aren’t absurd or unjust.
Trek has wobbled all over the place with these kinds of issues through every iteration. The problem is creating a dilemma for what are supposed to be military officers that doesn’t imply that at least one party is both an arsehole and shouldn’t be an officer with access to the weapons of a starship. The result adds to the wildly inconsistent portrait of Star Fleet as an institution and as a place where the chain-of-command is on one hand sacred and on the other hand a more of a vague custom rather than something enforced.
Meanwhile, the episode moves things along with whatever mysterious thing is going on with Georgiou and also moves things along with Adira and Stamets. The bridge crew also get new toys. None of which are big plot points (yet) but do demonstrate that the show continues to improve in giving a sense that everybody on the ship aren’t just holograms that wink off when Michael leaves the ship. A short scene with Tilly and Grudge likewise does a lot of this work. You don’t need big speeches or even a B-plot to ensure that a show about a crew of a spaceship feels populated.
ETA Cora makes the valid point that while the slave-worker story is a familiar one, it’s not one that is common in Star Trek http://corabuhlert.com/2020/11/21/star-trek-discovery-goes-on-an-unsanctioned-mission-in-scavengers/