At its center, The Empty Grave is a novel about friendship. Lucy works with her friends, mostly George and Lockwood, to fight against evil spirits who are determined to wreak havoc on their community. Individually, they can't defeat the evil spirits; as a team and as friends, however, they are. Friendship brings people together, as it does for Lucy and Lockwood and Co. Friendship is, as Stroud argues and shows, one of the most significant things a person can have in their life.
Unlike many novels written for children and young adults, The Empty Grave is a thematically rich and complex novel. It explores themes of the negative impact spirits have on people, the bravery of people who are in difficult and violent circumstances, and how family history impacts future generations. Not only that, Stroud's novel explores themes of death, psychic powers, and perhaps most importantly, trust.
As with previous entries into the "Lockwood and Co." series, The Empty Grave is told from Lucy's perspective. Lucy and the rest of the gang at Lockwood and Co. are one day hired to investigate if Marissa Fittes is interned in the prestigious Fittes Mausoleum. But Lucy and the rest of Fittes and Co. run into trouble: namely, the supernatural. To that end, the group explores what they call "the other side" throughout the novel.
In her past life, Marissa was an incredibly powerful psychic. Because of this, Lucy has a strong connection to Marissa. And as a result of this connection, Lucy can quickly locate Marissa's body. After Lucy finds Marissa's coffin, they discover something startling: in place of Marissa's body, there is a wax mannequin in the coffin.
Meanwhile, Lucy and the rest of the people who work at Lockwood and Co. (including the so-called "whispering skull") must deal with what they see as the most important threat to Britain—the incursion of ghosts into the country. They also must contend with Penelope Fittes' (who is related to Marissa) attempts to destroy Lockwood and Co. while trying to keep incredibly evil ghosts away from London.
