As the Thirteenth Doctor settles into her new body, she reflects that this is not her thirteenth body. And indeed, she's not the first Time Lord to surpass the limit of thirteen bodies.
Accompanied by Tegan Jovanka, the Fifth Doctor descends in a lift down to the Cloisters, casually advising Tegan to mind the slider, before Nyssa and Turlough catch up. As the Doctor tells them more about the Cloisters, he finds the entrance to Ophiuchus' secret lair, hidden behind a hologram.
Ophiuchus cordially welcomes them all, noting that all he wants to do is uncover the secrets of regeneration for medical purposes, something that the Doctor's companions cannot find fault with. As the two Time Lords begin a debate about the experiments, Ophiuchus collapses, asking for the Doctor's help to trigger his regeneration. Pushed along by his companions, the Doctor imparts a tiny bit of his regeneration energy into one of Ophiuchus's machines, allowing the device to copy the biosignature and allow Ophiuchus to regenerate.
When the golden light dies, the now female Ophiuchus is pushed to leave Gallifrey by the Doctor. Grateful the Time Lady departs, noting that her research may one day aid the Doctor. After she departs, the Doctor is left to explain to Nyssa how gender works for Time Lords.
Change is the only constant. But it felt different for the Fifth Doctor.
- A Time Lords thirteenth incarnation is usually their final one.
- Ophiuchus regenerates into a female incarnation.
- Ophiuchus' clothing changes along with their body upon regeneration. The Doctor's clothing also changed after his first regeneration in TV: The Power of the Daleks, and his boots changed into shoes after his regeneration in TV: Logopolis.
- The Doctor and his companions visit the Cloisters. (TV: Hell Bent)
- Ophiuchus makes use of a metamorphic symbiosis regenerator in his experiments. (TV: Mawdryn Undead)
- The Doctor recalls the murder of Nyssa's father by the Master. (TV: The Keeper of Traken)
- The Doctor mentions Omega to Nyssa. (TV: Arc of Infinity)
- The Doctor warns that Ophiuchus' research could lead to the Time Lords granting the Master a new life cycle. They will offer one to him soon after, (TV: The Five Doctors) and eventually enable him to regenerate again when they resurrect him for the Last Great Time War. (TV: The Sound of Drums)
- Ophiuchus suggests that her gift will help the Doctor one day, a prediction that will come to pass when the Time Lords grant the dying Eleventh Doctor a new life cycle to conclude the Siege of Trenzalore. (TV: The Time of the Doctor)
- The Twelfth Doctor recalls how different the Fifth Doctor's regeneration felt, and quotes the freshly regenerated Sixth Doctor's words as "change, my dear." (TV: The Caves of Androzani)