An exciting original story featuring the 12th Doctor, as played on TV by Peter Capaldi, and his companions Brandon and Alex. The TARDIS is under attack. Alien plants have taken root in its corridors, strange flowering tendrils that threaten to rip the Doctor's ship apart. New companions Brandon and Alex venture into the heart of the TARDIS to discover the source of the infestation. Instead they find a secret door and a forgotten world. The Doctor has unfinished business with the Arborites, living trees with long memories. Centuries ago he fought to prevent a galactic catastrophe - but now his past has come back to haunt him in ways he never thought possible. Can the Time Lord rescue his young friends before they are destroyed by his past mistakes, or will Hirolth rise to leave utter devastation in its wake?