The Doctor parks the TARDIS on a cliff (again). They're on Phobos, Mars' moon, which suddenly has an atmosphere and lots of opportunities for extreme sports. The Doctor thinks that's kind of unusual, especially since there's a mad scientist, Kai Tobias, going around babbling about monsters. A human woman named Amy and her alien lover Farl are on the run and pretend to be on holiday. It turns out that Farl's species wants to keep the bloodline pure, and don't take kindly to Amy. Their own violent Fantastic Racism causes a lot of backlash, and Farl faces discrimination everywhere he goes.