The Tenth Doctor is tinkering and pottering around the TARDIS, wondering where to go next with his companion Martha Jones. He invites her to pick a date, choose a date — anywhere. After ruling out most of her suggestions, having been there already and usually involved in some way (e.g. the Mary Celeste, the Princes in the Tower (twice – in two versions of history), the building of the Pyramids), it's decided to explore the mystery of Stonehenge on Salisbury Plain. Martha is keen to know how they built it. The Doctor, however, is more interested in why and decides it is worth a trip.
As Martha prepares to grab her coat the Doctor has a thought, "To observe is to change." He explains that the arrival of the TARDIS could be intrusive and interfere in what they are doing, so instead of landing and having a look around, the Doctor decides on another course of action. Not sure about the exact date that work started on Stonehenge, he sets the TARDIS, lined up with the rising sun, to appear for just a moment, long enough to take a snapshot picture with the scanner, dematerialise and arrive a year later and do the same thing, creating a time-lapse series of images to see what's going on over a century or two.
The images start off by showing a field. Over time they watch the grass grow; on one occasion they see a man covered in mud and paint, dressed in animal skins, gathering roots and berries. For years nothing much happened then gradually, they witness the stones being moved into place, enormous lumps of granite, huge megaliths being erected around the spot according to an intricate pattern the wise men devised, lining up with the sunrise.
Unbeknown to the Doctor and Martha, the TARDIS' arrival had been witnessed. Mangor had seen it first arrive, a strange coloured standing stone in an empty field, with the sun around it, obscuring its shape and texture and colour. Every year at the same time, this "stone" had appeared. It was seen as a sign, a message from the heavens, an inspiration, always when the sun was starting its longest journey across the sky. In tribute they had bought their own stones to the area, only putting the final stone in place only when the strange "stone" failed to appear.
The final picture the Doctor and Martha took in the sequence sees the people bringing bunches of wild flowers and the men bowing their heads. Still none the wiser the Doctor reflects, "I just hope that we haven't done anything, you know – silly."
- Possible journeys considered include the Battle of Waterloo, Charge of the Light Brigade, opening ceremony of the 2116 underwater Olympics, the third moon of Templastagon Five (with its beautiful sunsets).
- Journeys mentioned include solving the mystery of the Princes in the Tower (twice) and meeting a man who built the Pyramids (well not actually a "man").
- The original short prose fiction featured in the Doctor Who Files series of hardbacks from BBC Children's Books was officially sanctioned by the Doctor Who production office.
- Martha mentions meeting Shakespeare and solving the mystery of his lost play. (TV: The Shakespeare Code)
- The Doctor says that he solved the mystery of the Mary Celeste and that it wasn't his fault. (TV: The Chase)
- The Doctor suggests taking Martha to the moon but reconsiders because they have "done that". (TV: Smith and Jones)