Tate and Penny's moms are BFFs, so they've been in each other's lives since before they were born. They were as close as sisters when they were little, but diverged a bit as they got older, each having their own universe of friends. And yet, each was always fully aware of what was going on with the other. Tate was present for Penny when Penny's dad died before her eyes in a rafting accident, which was good because Penny's mom Lottie sure wasn't. 

Now the moms have brought the girls into one household because Lottie is donating part of her liver to Tate's mom, Anna, and while they're alone in the house, they have to deal with their attraction to one another, as much as they seem not to want to. 

The will-they-or-won't-they, as you might guess from the title, goes on for a long-ass time. Folks who like a slow burn won't mind, but I did.