Petra Williams
Dr. Petra Williams was Professor Stahlman's personal assistant on the Inferno Project. When Greg Sutton was brought in as an advisor, Stahlman ignored him, insisting on using Petra as a go-between so he didn't have to talk to Sutton. Initially, Petra was firmly on Stahlman's side, and regarded Sutton coldly. However, gradually, Sutton was able to persuade her that something was very wrong with Stahlman and that Inferno was heading for doom and disaster.
At any other time, Petra would've likely been the one person who could've gotten Stahlman to listen to her; however by the time she was siding with Sutton and attempting to get her employer to stop the drilling, Stahlman was already regressing into a Primord, and had become more uncooperative and tempermental than usual, and did not listen to her. It was thus up to the Doctor to handle the fully transformed Stahlman when he attacked the other scientists. After the Doctor subdued the regressed Stahlman using a fire extinguisher, Sir Keith Gold took over and ordered the drilling halted, just in time.
During the course of their trying experience, she and Sutton developed a mutual attraction to one another after their initial mutual hostility towards one another, although it is unknown if the two ever dated following the incident.
Alternate Universe
In the alternate universe, Assistant Director Petra Williams was very similar to her counterpart, clashing with, but eventually warming up to, Sutton. When the alternate Earth became doomed as a result of the drill penetrating the Earth's crust due to Director Stahlman's reckless actions, Petra accepted that she was going to die due to the Doctor being unable to take anyone back with him or else risk creating a paradox.
She, Sutton, Section Leader Shaw and Brigade Leader Lethbridge-Stewart helped the Doctor battle the fully transformed Stahlman and his Primord minions and agreed to help him return to his own universe to prevent their alternate selves from sharing their fate. While Sutton and the Doctor went to the shed where the TARDIS control console was housed along with the Doctor's car, Petra, Shaw and the Brigade Leader went to the power station for Inferno's nuclear reactor, so they could restore power to the TARDIS.
The transformed Stahlman attacked the three, and was shot by the Brigade Leader. They then fled to join Sutton and the Doctor, but when the Doctor activated the TARDIS' controls, nothing happened. Realizing there must have been some error, Petra risked her life to run back to the power station. While she was there making the final adjustments, Stahlman recovered and attacked her, but was subdued once again when Sutton arrived and sprayed him with a fire extinguisher. Finally managing to successfully connect the power, she and Sutton then ran to inform the Doctor.
Petra watched as Shaw was forced to shoot the Brigade Leader when he demanded the Doctor bring them with them and risk the paradox, allowing the Doctor to depart. Even as he was teleporting away with the TARDIS console and his car, the final explosion rocked the world, and a gigantic wall of lava flowed towards the shed. Petra clung to Sutton as the three of them could only watch the lava's approach helplessly. Fortunately, in the other universe, the Doctor was able to ensure their sacrifice was not in vain.
