Tegan Jovanka:  Doctor , why ca n't you help them ?
The Doctor: Do n't you understand ? If I did , it would be the end of me as a Time Lord .
Tegan Jovanka: What do you mean ?
The Doctor: I can only regenerate twelve times . I have already done so four times .
Tegan Jovanka: So ?
The Doctor: Do n't you see ? Eight of them ; _ eight _ of me .
Tegan Jovanka: They want your remaining regenerations ?
The Doctor: It 's the only way to end their mutation .