Many of the Doctor Who stories are filmed in a quarry. This time, it's even more blatant. As the Doctor and Sarah start walking trying to decide where they are, a siren begins blaring. Sarah is the first to realize they are indeed strolling in a quarry and the siren is warning they are blasting. She warns the Doctor a little too late and they are nearly caught in the blast. Sarah is encased in rocks when she finds a fossilized hand. Initially thinking it might be from the Doctor, she passes out.
Sarah's grip on the hand does not release and as she's taken to the hospital, it goes along with her. The ring on the hand is active and infects her mind. She's driven by the command, "Eldrad must live". She grabs the hand and walks towards the nearest nuclear power plant. There the hand starts absorbing the radiation, and begins to regenerate, starting with it's finger, then starting to move. It's looking for more power.
The Doctor is successful in separating Sarah from the hand, but the ring falls from her hands and one of the technicians of the nuclear plant finds it. It infects his mind also and directs him to take the hand directly into the power core. An 'unexplosion' takes place and Eldrad's regeneration is completed. Eldrad provides a sad story of how 'he' and his home planet were betrayed. The Doctor agrees to take Eldrad back home, but not back in time as Eldrad had been encased in the rock face on Earth for 150 million years.
Once on Kastria, they learn the female looking version of Eldrad is not truly how he looks. With Sarah being the first template he came in contact with on Earth his regeneration was patterned from her. Now back on Kastria, he assumes his true form and it's revealed he was the one who betrayed his planet and was executed. Fearing his return, Kastria deleted all traces of their DNA so he would not be able to recreate the race, leaving him as the King of Nothing.
Returning back to the Tardis, Sarah complains of always being cold, shot at and hypnotized. When the Doctor spends more time working on the Tardis than listening to her she feigns an attempt to go home and packs her bags. While doing so, the Doctor receives a mental summons from the Timelords. So when Sarah comes back into the control room, the Doctor informs her she has to go home as he cant' bring her with. The Doctor regrets having to leave Sarah behind and tells her they'll meet again. He then drops her off in South Croydon, Hillview Road - her home. Upon leaving the Tardis, Sarah realizes missed his mark again, and she doubts she's even in South Croydon. Whistling, she heads home.
WHO: Sarah Jane Smith
DEPARTURE LOCATION: Earth, circa 1976
REASON FOR DEPARTURE: When the Doctor receives an important summons from Gallifrey, he realizes he can't bring her with him and he drops her off.