The Ghost Hunter series revolves around Roddy and Tessa Oliver, two ordinary children whose lives are turned upside down when William Povey, a shoeshine boy from the Victorian era appears in Roddy's bedroom as a ghost and appeals to him for help.
The main action takes place in Roddy and Tessa's village and school where the Ghost Hunter is active.
Eventually, the children realise that the Ghost Hunter is Mrs Croker, but only when she arrives at Roddy's house with her "ghost immobilising vapour" (GIV) with which she tries to freeze William, shrink him and put him in a bottle.
Mrs Croker is also, and always, on the look out for other ghosts and has a whole collection of bottled ghosts that is discovered by Roddy, William and Tessa when the two boys save Tessa after she is kidnapped by De Sniff and left tied up and gagged in a secret hideout while Mrs Croker and De Sniff go back to the house to look for William.
Thanks to Roddy, Tessa and William, Mrs Croker falls from the battlements of Chillwood Castle, and is believed to have been killed.
She has survived her fall from the castle and returns to work in a strange old mansion, called Deadlock Hall.
Here, her demonic energy kicks into action again and she creates a machine called a "specktrika" which pulls ghosts towards it like a magnet.
Bex and Leo, having gone through the time portal to 1890, help a Victorian baby (Albert Povey) who is seriously ill, by bringing him back to 2027.
Their father, Dr Roddy, gives the baby antibiotics which saves its life, but by doing so, changes the course of history.
A final showdown between the pair sees DeSniff donating all of the money to the charity which has been set up and is celebrating Albert Povey Day.
Bex and Leo return to the future and in doing so, the time portal closes, perhaps forever.