Tanner was on the train to Chicago, before she and everyone else on the train were taken over by alien prisoners who had escaped from SAR TOP. The species that Tanner was is unknown, but was likely a Vardian. The crime he/she committed is also not revealed.
InformationEditTanner is first seen with Zin and Vax, confronting Adam Curtis about his failure to kill Troy Montana (the man from the 'Cole' underwear adverts whom Cole/Daggon took the form of). When Zin offers him another chance but warns of a small sacrifice, Adam accepts - only for Tanner to shoot him several times in front of a CCTV camera; declaring her dislike for Nodulians after the killing. Leaving the Nodulian inside Adam to die, they receive the video footage and edit it to make it look like Troy Montana was the murderer. Due to his likeness, Cole is wrongly arrested and charged with Adam's murder.
In the courtroom, the video is used against Cole to try and get him sent down for twenty years or more, with Zin knowing that with Cole out of the way, nothing can stop him or the other escaped criminals. Tanner, meanwhile, appears on the jury - although Cole senses her but cannot do anything about it at the time.
When Cole later escapes and switches places with Troy (who has returned to Chicago and been helped by Mel), he manages to defeat Vax and learn the tape's location. Taking it, he is able to discover that Adam's killer is the woman on the jury and adjusts the tape to show the truth. At the court, Tanner rings Zin to inform him of the progress been made, although admitting that 'Daggon' is acting strange. Needless to say, she ensures Zin that the ruling will be in their favor.
Having finished sorting out the tape and taking on Vax's form to avoid been noticed, Cole goes into the court room and gives the tape to Mel, who gives it in turn to her lawyer - Jonas. Managing to get the tape played despite not having presented it to the prosecution service beforehand (quoting other cases where such action was allowed because it directly refuted previously established events), the courtroom sees the truth after the video of the woman killing Adam is shown - much to Troy's delight as his innocence is proved. As Tanner tries to flee the courtroom, fighting those who try to stop her, Cole (still disguised as Vax) pins her down on a desk. When she asks why he has betrayed Zin, he reveals to her that he is in fact Cole. Tanner is then arrested for Adam's murder while the charges against Troy are dismissed.
Back at the Watchfire later, Mel and Jonas celebrate the court victory while Cole also thanks Jonas for his help - although Jonas still thinks Cole and Troy are the same person. That belief is shattered when Troy walks in and he and Cole meet for the first time - although Troy claims he can't see what people see that makes them look similar. When Cole asks Mel if he should start dressing like Troy, she immediately tells him not to.
It's certain that Tanner's life force was taken when Cole found a way to capture all the life forces at once, but it's not known if she returned when the prisoner's life forces began to return across North America.
Tracker (TV Series) - Episode 10: Double Down