So my friends and I are in the middle of watching the 5th doctor's era, and after watching POTD the other day and then watching Enlightenment (the episode after Nyssa leaves) today, I'm just struck by the tragedy of it all, of the sheer suffering Tegan went through - which, while not as cosmically awful as having your memory erased or getting trapped in a parallel universe etc, was nonetheless far too real and relentless.
So Tegan has possibly one of the most traumatic intro episodes. Unlike the overwhelming majority of companions, she doesn't choose to join the doctor. She enters the TARDIS while looking for help when her aunt's car broke down on the side of the road on the way to her new job, and since the TARDIS was parked around the Master's TARDIS due to the latter's trickery. The Master uses his tissue compression eliminator to kill Tegan's aunt, and Tegan gets lost in the seemingly endless labyrinth within the TARDIS, and when the Fourth Doctor and Adric return to the ship they take off with her still inside it.
She didn't want to leave Earth, and she spends the next few episodes begging the Doctor to take her back. In these episodes, she witnesses the Master destroy a quarter of the universe, watches the Doctor die and regenerate and is forced to look after him with Nyssa while Adric is kidnapped by the Master and the TARDIS travels back to the Big Bang and is almost destroyed. She carries the Doctor's recovering form through the jungle of a planet meant to be a sanctuary, only for it to be another of the Master's trap which they barely escape. She has to deal with Adric, and spends her first trip to a space ship trying to get back to the TARDIS so that she can get away from it all, and has to be rescued by the Doctor when this backfires. On the world of the Kinda she is possessed by the manipulative Mara and trapped inside her mind, an experience which would stick with her even after the Doctor saved her. She finally catches a bit of a break when they visit Earth in the past and have a relatively breezy adventure stopping Terileptils from trying to take over the planet (though they do inadvertently end up burning down London), and in the next episode she's decided that she doesn't want the Doctor to take her home anymore.
Black Orchid is a defining episode for the 5th Doctor's era. It is a light hearted romp and pure historical in the midst of tense action packed sci fi stories. Tegan eagerly watches the Doctor play cricket, orients Nyssa and Adric to Earth in the 1920s, and thoroughly enjoys her time at the costume party the TARDIS team is invited to at the local estate. Then it turns out a man who stole the Doctor's costume has kidnapped one of their hosts (who happens to be a doppelganger of Nyssa), the Doctor is arrested and Tegan and the others arrested as accomplices, and then Nyssa gets taken to the roof of the building as it burns and Tegan watches as her captor almost drops Nyssa off the roof and then falls to his death. They attend the funeral, and then head off again in the TARDIS.
And then we get to Earthshock. In the time leading up to this episode, the TARDIS team have become somewhat of a dysfunctional family. Though Tegan did not choose to join them on the TARDIS, she grows close to them all, especially Nyssa, sharing a room with her and bonding over their struggles with the new Doctor. Though she finds Adric annoying, she still cares about him deeply and bickers with him like one might a sibling. And she trusts the Doctor, though barely and with little respect, to get them safely from one destination to the next. Then in Earthshock Adric wants to leave, and Tegan doesn't get in the middle of him and the Doctor fighting, but they all quickly get distracted by the Cybermen plot they arrive in the midst of, and soon are all held hostage on a space ship headed to Earth with a cargo of Cybermen. The Doctor outwits them and they all end up back on the TARDIS except Adric, who is attempting to stop the ship crashing into Earth from the control room. While the Doctor fights a Cyberman inside the TARDIS, blaster fire damages both the TARDIS console and the controls Adric is working on, and so the Doctor, Tegan and Nyssa watch helplessly on the TARDIS scanner as the ship crashes into the Earth with Adric still inside. Tegan and Nyssa beg the Doctor to go back in time, to rescue him, but there's nothing any of them can do. Adric is dead - the only long term companion to die and stay dead while travelling with the Doctor.
Everything goes downhill from here. Tegan never quite recovers her trust in the Doctor after this. Immediately following Adric's death the grieving team arrives at Heathrow Airport, the work Tegan's been wearing her uniform for and trying to get to this whole time, and get dragged into yet another plot by the Master, one which involves them seeing a hallucination of Adric. And then they defeat the Master and Tegan gets to have a brief tour of Heathrow Airport before making it back to the TARDIS just in time for it to leave... without her.
Still reeling from everything she'd been through, Tegan nonetheless tries to work her air hostess job, but isn't able to cut it. After losing the job and trying to figure out her place back on Earth she decides to visit her favourite cousin who's on holiday in Amsterdam and arrives to find him having gone missing in a mysterious crypt. She tries to find him and gets kidnapped by Omega, a time lord and enemy of the Doctor, and used as a hostage against him (again). Even after that experience, once Omega is defeated Tegan is eager to rejoin the TARDIS team, to Nyssa's joy and delight.
The Mara resurfaces and Tegan relives her nightmares. A questionable new companion, the alien Turlough, joins the TARDIS and Tegan watches him warily, sure he's up to no good. On one particular day where Nyssa and Tegan are happily chatting together while Nyssa works on an experiment in their room, a portal opens up in their room and Nyssa gets drawn into it and ends up lost and sick on a leper colony ship. Tegan follows and gets lost in the ventilation system with Turlough. At the end of the adventure, after Nyssa has found a cure and the Doctor has stopped the space ship from exploding and ending the universe, Tegan is reunited with Nyssa only to receive the news that she's not coming back with them, she's staying on the ship to reform their healthcare system.
"She'll die here!" Tegan urges the Doctor to attempt to change Nyssa's mind but she will not be shaken.
"Not easily Tegan," Nyssa said, on the brink of tears. "Like you, I'm indestructable."
The two embrace and I honestly think Tegan never recovers from losing her. I believe Tegan would've stayed with her if she'd felt more confident with space and space travel, and she probably regretted not opting to stay behind many times going forward.
In the first episode after losing Nyssa the Doctor initially tries to get Tegan to remain in the TARDIS, in safety, but an Eternal takes an interest in her and she gets thrust into their games. She doesn't get a break, a moment to her herself to grieve, and instead has the Eternals violate her mind, recreating her and Nyssa's bedroom on their ship and keeping a close eye on her, using her for entertainment. The Doctor can't, or doesn't, do anything to protect her.
Now Tegan and Turlough do get along, but I don't think Tegan's heart is in it for the rest of her adventures with the Doctor. She travels with him because she has nothing else left, she's lost her aunt, her job, her closest friends. It's really quite tragic, especially considering that she's one of the longest lasting companions alongside Jamie, Clara and Yaz.
Having already suffered at the hands of the Master and the Cybermen, Tegan encounters the Daleks on Earth and sustains an injury that wipes her out for most of the adventure. She and the soldier nursing her spend most of the time hiding from daleks and trying not to get killed. Once the Daleks are defeated and Tegan rejoins the Doctor and Turlough at the TARDIS, she tells him that she's not coming with him, because she's seen too much death and travelling isn't fun anymore.
She seems to have said this in the moment, maybe out of desperation or in the hope that the Doctor would offer some comfort or promise of safety as opposed to the blank and broken stare of a man who had lost so much. So she leaves, but changes her mind at the last minute, running to the TARDIS just as it disappears for good. And that's the last she sees of him for almost forty years.
Until Power of the Doctor, when the Doctor walks in and barely even acknowledges her. Though I doubt she wants to travel with her again, she still never really got closure. She didn't choose to join the TARDIS and she was basically forced to leave, and then in the years after she would've heard of all the times the Doctor was back on Earth, and after she joined UNIT would've learnt of all the different incarnations and companions he'd had, yet she never came back to see her, after everything they'd shared, suffered together. She was convinced the Doctor didn't even care about her, and though she'd tried to live her life in a way that helped people, it had brought her right to this very spot, a fight with Cybermen, Daleks and the Master and a Doctor who barely looked at her and didn't ask her into the TARDIS. No wonder Tegan is so angry throughout that episode!
I don't even have a point to this I guess I just am thinking about her, and how Tegan deserved better, but I'm so glad she got her moment with the hologram doctor, an acknowledgement of everything they'd been to, and she got a place to discuss her trauma at the end with the Doctor support group. Also I don't care what Chibnall says, Tegan is NOT alone, her wife Nyssa is just busy at a space science conference and will be back soon to bitch about the Doctor.