On a remote and isolated planet, where technology cannot progress beyond steam-power, two thieves are on the run. Hunted and trapped by automaton policemen, they look certain to hang for their crimes... ...until Missy drops into their world. Their lives are about to change forever, as is the fate of their entire planet...

Missy needs to find the Doctor. However, it's very important it's the right Doctor - she wouldn't want to accidentally slip up and meet an earlier incarnation by mistake, because the consequences of that could be truly catastrophic...

Choices have been made. Fates are being forged. There are new friendships, new alliances, and a war to be fought, plus an extremely dangerous new arrival...