The first thing you should know about These Broken Stars is that it's a boy-girl book. That is, it's told through two perspectives, and one of those perspectives belongs to a boy and the other to a girl, and they alternate chapter by chapter. The protagonists here are Lilac and Tarver. Laura and Tom McNeal's Crooked is one of my favorite books that does this, alternating chapters between a female and male protagonist. There is also a section running throughout that details Tarver's debriefing. These short, one-page sections are stacked between each perspective chapter, and they largely keep the mystery of the story alive as Tarver blatantly lies his way through his debriefing. The reader can clearly see the story taking Lilac and Tarver in one direction, while Tarver makes sure to revise the narrative during his interview.
Tarver begins the story. He is traveling on board the Icarus, a huge spaceship that can hold up to 50,000 people. And it's packed to the brim. Although Tarver would never find himself on such a luxurious ship in any other moment in his life, his circumstances have changed recently and made him of interest to the rich, powerful, and influential. He is considered a war hero, even though he is still a teenager. He returned home a decorated soldier, and was given passage on the new ship as a reward. He observes those around him, and acutely understands that he still stands apart from everyone else traveling on the Icarus. For example, different from Lilac LaRoux, daughter of the most powerful man in (seemingly) the explored universe. When Tarver fails to recognize her as such, he ends up having a frank and genuine conversation with Lilac on board the ship. She's charmed by him much more than she expects to be, and the two find themselves drawn inexplicably to one another. Lilac is on board the ship because her father designed and built it.
The setting is gorgeous, especially when Tarver sets the scene in the first few pages:
For all their trendy Victorian tricks, there's no hiding where we are. Outside the viewpoints, the stars are like faded white lines, half-invisible, surreal. The Icarus, passing through dimensional hyperspace, would look just as faded, half-transparent, if someone stationary in the universe could somehow see her moving faster than light. (3)
Their time on the Icarus does not last. The book basically takes on the Titanic meets sci-fi, as the Icarus inevitably goes down, slipping out of hyperspace and crashing on a deserted, mysterious planet. Tarver and Lilac are the only survivors (Lilac saves them both by showing off her electrical skills when they are hurtling toward the planet in an escape pod). The planet is not like any other planet that Tarver has been on, and his military duty has taken him to plenty. It's not terraformed in the way that he's used to; in fact, growth on the planet has been accelerated, and there are plants and large animals that shouldn't be here. As they journey across the planet - seeking out the fallen Icarus - they find out that the planet holds a secret deeper and darker than they could have imagined, one that might prevent them from ever returning home.