Stralla Cushing
Hired by Cornelius Morningstar to build a working time machine on the Scapegrace space station, Stralla Cushing was noted to be the temporal scientist of her generation, and Morningstar harshly mocked her by saying she made Archimedes look like an Ogron when her experiments failed to work properly as she tried to create portal into the Time Vortex.
Possessing a nervous disposition, Stralla didn't like Morningstar shouting at her and mocking her all the time, and kept politely asking him to let her work. Morningstar, who wanted to use time travel for nefarious ends, threatened to cut her funding, which Stralla found unfair, and called him a 'big bully' behind his back while she was close to a breakthrough.
Eventually she managed to open a portal into the Time Vortex and rushed to tell Morningstar of the good news. Morningstar wanted to know if he could travel through time, though she said it was still early days and she still needed to do some research. Stralla didn't understand why Morningstar needed to bother opening time travel up as a tourism business since he was rich already.
At the time, the Eighth Doctor and Liv Chenka were travelling through the vortex looking for Helen Sinclair, who was with the Eleven, encountered temporal distortions in the vortex, forcing them to investigate and they shut down the gateway. When Cushing and Morningstar returned to the laboratory to find the gateway shut down, Morningstar flew into a rage, cutting off her contract and firing her, threatening to destroy her reptutation and Stralla snapped when she tried to convince him she could build a time machine and get it working. She killed him and shoved him through the gateway when she turned it back on to hide the body to stop the station security system tracking him down.
Investigated by the Doctor who had gotten suspicious of the event, Stralla managed to evade discovery but she was being blackmailed by Dron and Gorl who'd witnessed her murdering Morningstar, and they pressured her into giving her the time machine to keep quiet. She planned to kill them as well with a bomb she sneaked away from the security office.
The Doctor wanted to speak to her but got distracted while Stralla was desperate to get away, and the Doctor asked her questions about Morningstar. Stralla caused a distraction by setting off the fire alarm to get everyone away from the bomb when it exploded, but the explosion caused damage to the space station, and it nearly got destroyed by the nearby hyperspace gate when it was knocked off its axis and the time machine was heavily damaged, and the energy almost mixed with the hyperspace gate.
The Doctor could not understand why Stralla did what she did, believing her to be brilliant. Stralla almost left the Doctor behind in the chaos, when she confronted the Doctor asking him if he knew what she had done and she confessed her crimes - the murder of Morningstar, and the blackmailer's murder, and she just wanted to escape. The Doctor managed to point out he was the only one who could save her life, and she saved his life while he worked to save the station.
After the Doctor managed to prevent the station from being destroyed, Stralla tried to slip away under disguise, but the Doctor and Liv saw through the disguise with the help of a bio scanner and she was arrested. She kept making excuses, saying a shapeshifter had assumed her form, but that was seen through as well.
The Doctor watched her arrest sadly, noting that despite her murders Stralla was "such a waste," and noted Stralla had "an enormous brain, and a complete idiot." (AUDIO: How to Make a Killing in Time Travel)
