This book was illustrated at least by Piers Sanford (UK) and Tom Newsom (US). HarperCollins recommends its 2004 edition for ages 8 to 12. Kirkus recommended it for ages 10 to 13 in 1993. And concluded, "Not the best Cupboard book, but fans won't want to miss it; with a first printing of 75,000 they won't have to."
Kirkus observed that "Banks plots expertly" and develops the relationship between Omri and his father. "There's not much chance to stereotype Native Americans here, as Banks was charged with earlier, but Jessica Charlotte is certainly a caricature of a music-hall singer; one wonders whether it's reasonable, or merely foolish, to deplore such shorthand in popular fiction."
Omri and family move to the rural country, where they have inherited a house from the family of Jessica Charlotte Driscoll, Omri's "wicked" great-great aunt. There Omri learns the origin of the magic key and its history of time travel. Omri does all of this as one night when the roof is being re-thatched; he finds a cashbox and a journal called the Account in the fallen thatch on the ground. The Account was written by Jessica Charlotte herself as she was dying of a fatal illness. Omri learns that she has a psychic gift that grants her some magical power to see the future by pouring lead for people, although she was also a dance-hall star in the early twentieth century. After she is separated forever from her niece Lottie (Omri's maternal grandmother who was killed in the Blitz during World War II) by Lottie's mother, she steals a pair of precious earrings from her sister. To do so she makes the Key, a duplicate of the key for her sister's jewel-case. She accidentally imbues it with magical power through her own Gift, as she is so bent on making it, which allows her to open any lock with it. She is also accidentally responsible for Omri's great-grandfather's death: Lottie is accused of stealing the earrings and runs out into the street where her father follows her, resulting in him being run over and killed.
Unfortunately Jessica grows too weak to finish the Account and has Omri's great-uncle Fredrick takes over. From this Omri learns that Fredrick actually created the Cupboard, which is itself somewhat magic. Fredrick had a hatred of plastic toys- which had replaced the metal toys he used to make as a profession- so Jessica had him create the cupboard and throw his hatred of plastics into it. Since Fredrick inherited some of her power this caused the cupboard to somehow only bring plastic toys to life, which is why no other materials ever worked when Omri tried them in the cupboard. Omri's interference with his own family history sets the stage for his first adventures.
Omri also meets one of the thatchers who worked on the roof when Jessica was dying, a man named Tom, and learns from him that Jessica somewhat let him in on the secret. She also gave him a little person to care for on his own named Jenny, a servant girl from the 1800s who sought to escape her home life after she lost her position, but after having lived as a little person for thirty years she had died a few months before Omri met Tom; Tom speculates that, based on the timing of the era when she would have left, hospitals were becoming so overcrowded that the doctors simply decided to let Jenny's seemingly comatose body die. Omri initially suspects that the stolen earrings are in the cashbox he found with the Account and realizes that the magic key could probably open it. He goes to get it out of the bank during his half-term, during which time he also has Patrick join him. Patrick breaks his arm and has to go to the hospital; while he's away, Omri opens the cashbox and finds five wrapped bundles, four of whom are people brought to life by the magic key (although one has died since he was last 'summoned' and only his damp uniform is left). He suspects the fifth bundle is the earrings. The people turn out to be Elsie, a widowed shop owner; Bert, a thief from Elsie's time; Ted, a retired policeman from around twenty years after them; and (the one who had died) Sergeant Charlie Ellis, a naval officer who apparently fought at the Battle of Trafalgar; all of whom Jessica herself brought to life repeatedly with the cupboard, along with Jenny. They reveal that Jessica knew of the power of the key and the cupboard but only found out when she accidentally brought them to life for the first time. She brought them back and forth many times like Omri did with Little Bear and the others and eventually revealed to them that she was dying before sending them all back, except for Jenny who had only a horrible life to go back to. Omri learns that one of them, Bert, is the one who stole his great-grandmother's jewelry box after her husband had died and made her live a poor life. After learning this (Bert had only just done it before he was brought forward, although he defends himself on the grounds that he believed that he had robbed the woman before her husband had died), Omri insists that Bert return the jewelry box to her hoping to change her future for the better. He finally convinces him and then sends them back one final time through the cupboard.
Omri subsequently learns that the last bundle is not the earrings but another sleeping person, but before he can investigate further he is called to the house of the old thatcher Tom. Tom fell and was fatally injured but asked for Omri, having remembered something more. He tells him that Jessica sent him all over the place looking for a plastic figure of herself. He bought dozens not knowing which one was just right and after she looked through them and found the one she told him to take care of the rest, although still just plastic figures, saying, "This is me but everyone is someone". After he finishes his story Tom loses consciousness— his last words being "On my way, Jen" — and is taken to the hospital, where he dies without ever waking again. Omri and his father also retrieve Patrick from the hospital and head home where Omri explains that he's figured out the identify of the final person he brought to life: Jessica Charlotte herself, apparently just after she fell asleep after creating the key.
Omri wants to convince her to not to take the earrings but Patrick tells him that doing so could change the future and possibly result in him not being born due to Lottie's life turning out in such a manner that she never meets his grandfather. Omri freaks out, remembering that he convinced Bert to return the jewelry case, and worries that it will cause him to cease to exist. Jessica wakes up and Omri and Patrick tell her she's visiting the future and it's a dream and get her to give a performance for them. They greatly enjoy it, but when Gillon starts to come in they send her home suddenly through the cashbox. Later Omri learns from his mother that the jewelry box was returned in ruins with none of the jewelry inside, changing nothing (Bert only said he would return the jewel-case without specifying that the jewels would still be in it). He then remembers she mentioned something about this back when she first gave him the key so everything is alright. A week later he attends Tom's funeral and figures out that Jessica gave the earrings to Tom who gave them to his daughter after Omri sees her wearing them. He's solved the mystery but decides not to do anything about it.
He returns home to find that his father has added proper shelves to his room and bolts on the door so he can lock them. He's excited but can't find Little Bear and the others. He goes to his father to ask him about it and learns he locked them in the cupboard and heard them come alive. In the end Omri's father finds out about the "little people" and is let in on their adventures. He promises not to tell anyone and Omri brings him in on the secret and introduces him to everyone.