In a psychic forest the Master begins a sinister experiment... but is it for the benefit of Gallifrey or does it serve a darker purpose?
Several years ago, the Master, disguised as a tutor, took a group of Time Lord Academy students with promising futures on a field trip that went disastrously wrong. Now in the present, they all turn up dead.
Gamara is the last survivor - but to save herself, she will need to reveal the truth about the monstrous experiment the Master made them conduct all those years ago.