Shadows Of Avalon — Paul Cornell

The Brigadier, mourning the loss of his wife Doris, is called to help find a nuclear weapon that's gone missing over the Wiltshire Downs. The Doctor is on his way there too, to pick up his companion Compassion, after her holiday on Earth. But when the Doctor's TARDIS explodes, he, the Brigadier, Compassion and Fitz are thrown into the other-dimensional world of Avalon. Magic faces down science, dragons duel with jet fighters.

Originally published in 2000.