What secret is the TARDIS hiding?

Enlisting UNIT in their search for an enigmatic woman who appears all throughout time and space, the Doctor and Ruby uncover deeper mysteries. What is the secret of Susan Triad? What happened on the night that Ruby was born? The answers lead the Doctor and Ruby to a horrifying confrontation with the greatest evil of all...

Russell T Davies' epic two-part finale of Ncuti Gatwa's first season as the Doctor is novelised by series script editor Scott Handcock, reinstating material from earlier drafts.