The 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.