At the start of The Hollow Boy, Lucy, Lockwood, and George (who comprise the ghost fighting company Lockwood and Co.) are sent by a government agency called the Department of Psychic Research and Control (DEPRAC) to investigate several disappearances at a boarding facility. After they arrive at the boarding school, its owners initially don't want to allow Lockwood and Co. into their house. But eventually, Lockwood talks the owners into allowing them into the house.
The group begins to investigate the disappearance, which the couple doesn't like. After Lockwood and Co. enter a room, the couple slams the iron door closed on them, locking them in the room with ghosts and other demonic spirits. The group works hard to escape from the boarding house and the room; they can get away from the room, but come face-to-face with an evil being known as the changer. The group escapes and Scotland Yard arrests the couple, leaving Lockwood and Co. to contend with other ghosts plaguing London.
When she goes on cases, Lucy frequently consults with the mystical and all-powerful Whispering Skull. The Skull is wise and resourceful and is a boon for Lucy. That's why, when her agency gets overwhelmed by the sheer number of jobs that are coming in, the Whispering Skull makes Lucy's life easier.
Lockwood and Co. get involved with the hauntings which are wreaking havoc all over the Chelsea section of London. The kids in Lockwood and Co. work together to solve the hauntings in Chelsea. Along the way, though, they encounter dangerous supernatural beings which shake them to their very core. These experiences cause Lucy to take a vacation. She goes to a relatively remote part of England to visit her family, each of whom she loves.
While in England, she initially relaxes. However, a woman named Mrs. Wintergarden encounters Lucy and asks her and her agency to solve the haunting of her house. She is not a nice woman, but Lucy and her agency agree to take the case. They deal with and ultimately solve, the haunting of Mrs. Wintegarden's home—all while contending with the worsening outbreak in Chelsea in London.
Ultimately, the team is hired to get rid of a poltergeist. And though it gives them quite a bit of trouble (and constantly threatens their lives), they are eventually able to defeat it with the help of the namesake hollow boy, who is an enigmatic figure. The book ends with the kids in Lockwood and Co. getting to know the open boy, who helps them resolve the Chelsea outbreak.
