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The Doctor finds himself too weak from the poison's effects to pilot the TARDIS to rescue his companions; River is inspired by the Doctor's sympathy, and finds herself guided by the TARDIS itself to pilot the ship, and rescues Amy and Rory in time. On returning to the café, the trio finds the Doctor near death; he asks River to find "River Song" and give her a message, then whispers something in her ear before he passes away. River, who at this point still only knows herself as Melody Pond, asks Amy who River Song is; Amy uses the Teselecta to show River her form stored in the robot's database of who she is to become. With this, River sacrifices her remaining regenerations to bring the Doctor back to life, and passes out.
From tardis.wikia
Regenerative cycle
Time Lords had a limited regenerative cycle of twelve regenerations, consisting of thirteen incarnations, after which they would suffer permanent death. (
DW:
The Deadly Assassin,
Doctor Who) Time Lords could will themselves to die by regenerating when they had no more regenerations left to use, as
Azmael chose to do. (
DW:
The Twin Dilemma) Rassilon apparently had physical reasons to impose this restriction. (
BFA:
Zagreus)
As with most such "rules" there were occasionally exceptions to the twelve regeneration limit. The
High Council, offered
the Master a new regenerative cycle if he rescued the various incarnation of the Doctor from the
Death Zone. (
DW:
The Five Doctors) Although he did not receive this award on that occasion, during the
Last Great Time War the Master was given the ability to regenerate at least once more. (
DW:
Utopia)
In his
Eleventh incarnation, the Doctor claimed he could regenerate five hundred seven times. (
SJA:
Death of the Doctor) However whether the Doctor was speaking the truth is unknown.
Time Lords could be revived with regeneration energy. Doing this,
Melody Pond (in her third incarnation) used up her remaining regenerations to save the Doctor's life and was hospitalised as a result. It is unknown if this act gave the Doctor her remaining regenerations. (
DW:
Let's Kill Hitler)
I guess it would just be Jack's blood, but it confuses me why they didn't choose Superman's blood, or some other person who couldn't die. LOL.
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