SARAH: Doctor, listen. Some vast impulse of energy has drawn the TARDIS off course. The UNIT house was built on the site. Yes, it is the same signal.
Plan: A: The Doctor; Q: Who is drawn off course with Sarah? A: UNIT HQ; Q: Where were the Doctor and Sarah supposed to land? A: 1911; Q: What year do the Doctor and Sarah arrive in? Summary: The Doctor and Sarah are drawn off course and, instead of UNIT HQ, they arrive on the same site in 1911.