We live forever, barring accidents. Just like everyone else in the universe.
The Doctor travels back to the Ancient Days, an era where life flourishes and death is barely known...
Then come the Kotturuh - creatures who spread through the cosmos dispensing mortality. They judge each and every species and decree its allotted time to live. For the first time, living things know the fear of ending. And they will go to any lengths to escape this grim new spectre, death.
The Doctor is an old hand at cheating death. Now, at last, he can stop it at source. He is coming for the Kotturuh, ready to change everything so that Life wins from the start.
Not just the last of the Time Lords. The Time Lord Victorious.