The TARDIS takes the Doctor and Freja to the planet Ampersand Thrax, and the site of a scientific outpost run by the HEF – the Human Expeditionary Force. They are on the edge of the known universe, trying to understand the science behind an ancient celestial body – a biosun – in the hope of ensuring the future of the human race. As the Doctor questions how much he can help and Freja investigates what’s inside the life support room, the travellers make enemies both inside and out.

The planet is inhabited by the Fer-Kyste, harpy-like creatures able to generate bursts of fire at will, and they resent humanity’s presence on Ampersand Thrax. With only mechanised mining suits to defend themselves, the HEF are fighting a losing battle, under near-constant attack from these pyrokinetic aliens. But an expedition to their nest-city high in the mountains will change everything. As Freja struggles to keep her new friends alive long enough to complete their important mission, the Doctor is pushed to his limits as he must decide how far he is willing to go to save the lives of those who would see him dead and discover whether even he can survive the flame of the Fer-Kyste…