Doctor Who fans are used to bidding beloved characters farewell. From Doctors to companions to even the villains we love to hate, seeing them go can be bittersweet. This is especially true in the modern series, where companions seem to die much more frequently than they did in the original show. However, since many of the classic series companions simply decided to leave and return to their lives, their stories were left somewhat open-ended. Much like Thirteen's companions Graham, Ryan, and Dan, they were simply done. They'd had their adventures, and were content to live out the rest of their lives with their memories of all of time and space.
Tegan and Ace, however, were among the classic series companions who parted from the Doctor on less than stellar terms. Tegan traveled with the Fifth Doctor and had a falling out with him. Ace was a companion to the Seventh, whose manipulation of her resulted in the end of their friendship. For the first time in decades, both these companions appeared onscreen in the latest special, working for UNIT and Kate Lethbridge-Stewart. They're beloved companions whom longtime fans were ecstatic to see again — but their inclusion in the story wasn't just to get classic series fans' blood pumping. “The Power of the Doctor” not only featured Ace and Tegan in roles that were important to the story, but allowed them each closure with their respective Doctors.
Their Departures
In the 21st-season story "Resurrection of the Daleks," Tegan had a falling out with the Fifth Doctor (Peter Davison) after a four-part serial of fighting the infamous tin-can monsters. As the TARDIS is about to leave, she tells him that she doesn't enjoy traveling with him anymore, and runs off without a proper goodbye. The Doctor, clearly upset, leaves with his other companion, Turlough. Tegan appears to have a change of heart, but the TARDIS is gone by the time she runs back. As viewers can see by her appearance in "The Power of the Doctor," she regrets leaving somewhat unceremoniously, but is also hurt that the Doctor never cared to check in on her again.
Ace’s departure from the Doctor's company was messy, because it took place during what was known as "the Wilderness Years." She was still traveling with the Seventh Doctor (Sylvester McCoy) at the end of the classic series, but by the time of the 1996 TV movie, he was traveling alone again. Expanded media built upon the story of their separation, and although it's hard to say what's canon and what's not, the general idea is that the Seventh Doctor had been manipulating parts of Ace's life. Understandably, she was upset when she discovered this and left the Doctor's company with some rancor hanging between them. She briefly touches on this when her hologram activates and the Seventh Doctor appears to her, apologizing to him for their falling out. Yet, she doesn't seem quite as bitter towards the Thirteenth Doctor as Tegan is; rather, much like her younger self, she's eager to get back into the action.
The Return
Tegan and Ace reappear on Doctor Who in the perfect context for two former long-term companions: Kate Lethbridge-Stewart has recruited them as consultants for UNIT. Together, they investigate the disappearance of famous paintings and seismologists around the world — until Kate brings them in to reunite with the Thirteenth Doctor.
Their reactions to seeing their old friend are perfectly in line with both their characters, and the situation at hand. They don't immediately fall into the Doctor's arms as if nothing happened between them. Both Tegan and Ace meet Thirteen with equal parts awe and anger befitting the circumstances of their departures. Yet, despite their personal feelings, they both know the stakes at hand, having been in Earth-shaking situations before. As Tegan attempts to rescue Kate from the Cybermen and Ace delves into the volcano to stop the Daleks, they each get a chance to confront their Doctors in holographic form. Even though the Fifth and Seventh Doctors may be long gone, Tegan and Ace each get a chance to say the goodbye they never got. They each get a chance at reconciliation for what happened decades ago. And in getting that closure, they each find the strength they need to help stop the Master's plan.
Often, the Doctor's companions leave the show and are just...gone. They may appear for an occasional cameo, or star in a spinoff like Sarah Jane Smith. However, this is more or less a concept unique to the reboot, and was not explored in the classic series. Companions of the first seven Doctors were more or less written off, never to be seen again. Most of them left the Doctor's company of their own free will, but surely they didn't walk away as the same people who first stepped into the TARDIS. As Graham and Dan discuss in their newly formed support group, their experiences changed them forever.
As audience members, we don't often get a chance to see what happens to the companions after they return to everyday life. "The Power of the Doctor" changed that. It could have done so with characters who had less messy departures from the show. Some characters would have simply gushed to see the Doctor again, and followed along with her plans without confronting any complicated feelings. But by reintroducing these two particular companions, Doctor Who examined how companions' adventures through time and space can result in not just positive memories, but also traumatic ones. And by allowing Ace and Tegan to address that, "The Power of the Doctor" brought the stories of these two beloved characters to satisfying conclusions.