There is an arch space ship currently in orbit around a planet that shows signs of alien life. As the ship is already overpopulated, there is an age limit for those that didn't make it into the science fields. The methods of death are either a quick, painless death through injection so that their bodies can be used to fertilize the crops or assignment to a high fatality work shift. Some choose to jump out of an airlock before the choice is made for them, though their bodies are collected and restored enough to add to the fertilizer group.
Sally is three years below the age limit at the start of the book. She's assigned a task in the back of the ship and takes the I-beam, a somewhat fun express ride along the top of the ship. Harry lives near the back of the ship, not too far from where the assignment is. He's feeling rather down at the moment, as his birthday, the birthday, is coming up in a couple of days. Neither had ever met that special someone, and both are worrying that they never will before their time is up. Then they meet each other and they are that for each other.
Kind of a Logan's Run meets When Harry Met Sally. They will probably try to get a drop pod and head down to the planet, trying to stay out of the way of the locals. But that has its own level of uncertainty.