The titular character was a loopy witch who got all her power from endless cups of tea brewed from her magical, evil tea plant by her reluctant sidekick (or T-Caddy) called T-Shirt, played by John Hasler.

T-Bag would take up residence inside some item in T-Shirt's Grandad's curiosity shop (e.g. a book, music box, boardgame, etc.) and oust whoever else was living there. This usurped character would often have the means to banish T-Bag in the form of a number of mystical objects (e.g. letters, numbers, bells…), that T-Bag would often steal and scatter throughout the magical world inside the object in which she was living. A young girl (of which there were five over the run of the series) would then enter the shop and her attention would be drawn to the usurped character in the object. She would get summoned into the object by this character and then embark on the quest to recover the objects and ultimately banish T-Bag. There were often variations to this storyline, but it generally applies to the whole lot. Elizabeth Estensen (The Liverbirds, Emmerdale) played the original T-Bag (Tallulah) for the first five series, to be replaced by Georgina Hale (One Foot in the Grave, Preaching to the Perverted) in the subsequent four series as Tabatha Bag, Tallulah's older sister.