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The series particularly delved deeper into the history and character of Alistair Lethbridge-Stewart, creating a whole family for him not previously seen on television. Family members included; Mary Gore, his mother, James Lethbridge-Stewart, his older brother who died in 1938, and Gordon Lethbridge-Stewart, his father, a commander in the RAF who was declared Missing In Action at the end of World War II. It also debuted his children and grandchildren with Doris Lethbridge-Stewart, the Wilsons (son, Albert, daughter-in-law Tamara, and grandchildren Conall, Nick and Lucy). Lucy would later head her own series of spin-off novels, with guest appearances from various members of her family.
Along the way the series also delved into the Brigadier's ancestors and extended family, including cousins, uncle, and grandparents. Most notably in the short story anthology, Lineage.
The series also developed an extended family for Anne Travers and her father, Professor Travers. These included; Alun Travers, Anne's brother. Lyndon Travers, Anne's grandfather and Professor Travers' father. Margaret Travers, Anne's mother and Traver's wife, and Travers' twin, Vincent and his children. Ben Travers, from Horror of Fang Rock, was revealed to be the uncle of Professor Travers.
In addition to the novels, which initially followed an ongoing narrative, the short stories occasionally dipped into different periods of Lethbridge-Stewart's past and future. For instance, Ashes of the Inferno was set directly after the TV story, Inferno, and The Lock-In was set around the time of his death, with Of the Future exploring the divide between the Brigadier and his daughter, Kate, and the birth of his grandson, Gordy. The Enfolded Time brought focus on the UNIT dating controversy and could be said to finally solve the longstanding dating issues once and for all. Though the regularity with which spin-off media attempts to "fix" the controversy makes it one of many.
From late 2018, with The Laughing Gnome series of books, the range moved from the ongoing narrative that featured in the first fifteen novels, and began to dip into various periods of Lethbridge-Stewart's past, present and future.