Natalie Lang

 was the leader of the ship that was testing a time travel technology which crashed into the TARDIS. When the First Doctor discovered that the ship was going to be used as a weapon in a war she wanted to end, she shot Steven Taylor.

When she discovered that they were in the TARDIS, she wanted to use it to get it back to her own time, but instead caused it to overload. (AUDIO: The Anachronauts)