Dominic Sandbrook concludes his exploration of the most innovative and imaginative of all genres by considering science fiction's most alluring theme - time travel.

Having the power to change the past or see the future is a deep-seated human fantasy, and writers and film-makers have embraced its possibilities. From HG Wells's pioneering scientist in The Time Machine to Back to the Future's Marty McFly and Doctor Who's titular Time Lord, we've been presented with a host of colourful time travellers and their time machines. But is there always a price to be paid for meddling with the timeline?

Among the contributors are David Tennant, Karen Gillan and Steven Moffat (Doctor Who), actor Christopher Lloyd and screenwriter Bob Gale (Back to the Future), actor Scott Bakula (Quantum Leap) and novelist Audrey Niffenegger (The Time Traveler's Wife).