The book, loosely based on one of the author's own experiences, features an expecting calico cat mother who wanders off into a ramshackle house, having followed by the howls of an old, chained up hound. She befriends the dog, named Ranger, and has her kittens in the Underneath, a hideout under the porch of the house. She names them Puck and Sabine, and they are warned not to leave the Underneath.
Unfortunately, Puck does just that, and his overt curiosity provokes the attention of Gar-Face, the cruel hunter who resides in the ramshackle house.
Meanwhile, Grandmother Moccasin, a lamia, awakes after a thousand-year rest, and laments her past, involving her only daughter, Night Song, being lost to Hawk Man. All the while, her "brother", the Alligator King, is the prime target of Gar-Face
With themes of loss, love, and mystery, the book is filled with Tear Jerkers.
Tropes found in this work include:
- Abusive Parents: Gar Face's father, who scarred his face, thus his name.
- Afterlife Antechamber: The hummingbird AKA Night Song and Hawk Man's daughter serves as this, as the messenger to the recently dead who guides them to a vague afterlife.
- Bridge Logic: Puck is able to escape at one point using a falling tree as a bridge (the same tree that Grandmother is buried under.
- Curiosity Killed the Cast: One involving literal cats! Puck sneaking outside of the underneath triggers a lot of horrible things happening to him and his family later on.
- Cute Kitten: Puck and Sabine, though their cute antics don't end well for them.
- Cruella to Animals: Gar Face is a bit too happy about abusing and killing various animals.
- Death by Despair: Night Song after she is unable to see Hawk Man thanks to Mode Lock.
- Emergency Transformation: Subverted when Hawk Man is slowly dying form grandmother's venom, but refuses to shape shift to heal himself because it would Mode Lock him into a hawk and thus allow him not to see Night Song again, since he's still holding out the hope that she will come back.
- Exit, Pursued by a Bear: Gar Face ends up being eaten by the alligator king when he tries to kill him.
- First Time in the Sun: Puck sneaks out and is elated to be able to bask in the sunlight for the first time.
- Freudian Excuse: Not quite an excuse, but Gar-Face is somewhat sympathetic due to his face being scarred by his abusive father. Not much of one, though, because even before that he poisons and kills birds just to torment his mother.
- Happily Adopted: The kittens by Ranger, and Night Song by Grandmother, at least at first.
- HeelFace Turn: A very heartwarming one. "Grandmother, who had spent a thousand years in a jar, had finally chosen love. This time she did what she could to help it along." She'd broken the chain tying Ranger to the tree.
- Human-to-Werewolf Footprints: Hawk Man discovers Night Song's footprints change when she transforms back from a human into a snake. Given that she can only turn into a human once in her life...
- Ironic Echo: Throughout the book, Grandmother Moccasin repeats to herself, "There is a price." In the end, she realizes the price isn't a punishment for her betrayal. The price was sorrow, sorrow for interfering with love. Before she dies, she realizes that her words are true, but not in the way she thought.
- Kid Hero: Puck and Sabine.
- Knight Templar Parent: Grandmother tries to keep her daughter Night Song away from Hawk Man believing that she is protecting her, but to accomplish this she tricks her into transforming and thus never being able to see Hawk Man again, with tragic results.
- Mode Lock: All shapeshifters can only transform into a human and back once, after which they are stuck in their animal form permanently.
- Mutual Kill: While not strictly deadly for one side, Hawk Man manages to lock Night Song in a jar while being hit by her venom.
- Nearly Normal Animal: Most of the cast is this.
- The Nameless: The name of the calico cat is never revealed.
- Never Smile at a Crocodile: The Alligator King, who manages to devour Gar-Face whole.
- Oh, Crap!: Puck and the calico when they realize Gar Face is trying to take them to the water.
- Orphan's Ordeal: Very much so. Puck goes through a lot after losing his family with one of his parents being dead, as well as Hawk Man and Night Song's daughter.
- The Place: The titular underneath.
- Redemption Equals Death: Grandmother breaks Ranger's chain to save him and ends up getting killed in the process.
- Sealed Evil in a Can: Grandmother has been trapped in a jar for a thousand years.
- Serpent of Immortality: Grandmother is a lamia trapped in snake form who is thousands of years old.
- Snakes Are Sinister: Grandmother at times, tricking her daughter into transforming permanently into her snake form to keep her away from Hawk Man, though she regrets her daughter's resulting death and ultimately breaks Ranger's chain. Night Song, on the other hand, is clearly not sinister.
- Star-Crossed Lovers: Night Song and Hawk Man. Grandmother is not happy with their relationship.
- Stronger with Age: Why the alligator king is so huge.
- The Outside World: The kittens are very enthusiastic to explore outside the titular underneath, despite the dangers.
- Three Lines, Some Waiting: The plot is split between the family at the Underneath, Puck, while he is alone after Gar face tries to kill him and Grandmother's story from a thousand years ago, which only connects to the main plot much later in the story.
- Wise Tree: All trees are sentient and wise, especially the thousand-year old tree that Grandmother is buried under
- You Have Failed Me: When Ranger refuses to kill a bobcat because she had kittens, Gar Face shoots him and then chains him in his yard.
- Xenofiction: Pretty much all of this book is from the perspective of animals.