The Eighth 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.
Kai turns out to be right: there is a monster, and people are dying. The local "Drennies" (adrenaline junkies) don't care, since they're in extreme sports paradise and just want to go bungee jumping.
Farl lashes out violently when he feels that his relationship with Amy is threatened. Amy, now becoming fast friends with Lucie, becomes scared about what their relationship might turn into — especially now that she's pregnant.
Eventually, the Doctor finds out that the interdimensional portal on the mountain houses a monster that feeds on happy fear: adrenaline. Specifically, he finds out when Kai is about to throw him and Lucie into it. The monster has shaped Phobos into an extreme sports paradise on purpose so it can feed. Eventually, it'll break through the rift and become a real threat. Kai knew that the beast is harmed by actual, pure fear, which is why he started the exaggerated myths to protect people and subdue the monster... and when that failed, he started killing people just to drive the Drennies away. When that failed, he set Farl up to become violent and frighten Amy. And when that failed, he figured that dropping the Doctor and Lucie down the portal might create enough fear to do the trick. This earns him a grand What the Hell, Hero? from the Doctor.
The monster possesses Kai's girl, Eris, and starts talking to the Doctor: explaining to him that his love for saving people, his excitement at rescuing the weak in fearful situations, has fed it to the point of being able to break through the rift. His own adrenaline rush of being the big damn hero has only made the monster stronger, and will become the cause of the deaths of thousands. The Doctor counters that he would never become that sort of person and decides to take matters into his own hands. By taking the Drennies' equipment and bungee jumping into the interdimensional portal.
And he conquers the monster effortlessly by showing it all of his fear. He starts by feeding it detailed memories of all the things he's seen in the past, followed by all the evil he's seen from the future... and as a magnificent final blow, all the things he's afraid he might do one day. It takes several minutes, with the Doctor continuously mocking the monster throughout. Eris is killed in the process.
In the aftermath, Kai buries Eris and tries to become a better person; two of the Drennies are prodded to finally admit their man-love for each other; Amy and Farl decide to slowly re-evaluate their relationship, and welcome their new baby into their lives.
The Headhunter has been on Phobos the whole time ... unconscious due to an accidental head injury. Having missed Lucie again, she is getting exasperated.
- Big "NO!": The last thing the entity spouts.
- Bottomless Pits: The Wormhole is theorized to be one.
- Call-Back: Lucie nearly goes over a cliff, just like in My Immortal Beloved.
- Future Slang: "Drennies" for "Adrenaline Junkies" which litter Phobos.
- Emotion Eater: Mainly fear but with a side of pleasure in experiencing the fear.
- Foreshadowing: See also Sequel Hook.
- Gravity Screw: Phobos, the moon, has an odd gravitational pull due to its ellipse shape.
- He's Not My Boyfriend: Lucie about the Doctor.
- Heterosexual Life-Partners: Drew and Hayd appear to be this, but it's subverted with the reveal that Drew has suppressed feelings towards Hayd.
- Interrupted Declarationof Love: At the end of the story, before Drew can confess his feelings towards Hayd, he is interrupted by the Headhunter.
- List-of-Experiences Speech: The Doctor lists what he's seen that's left him jaded to the monster's threats.Ive seen entire species destroyed, civilizations left in ruins. Ive witnessed solar systems vanish in the twinkling of an eye. Ive seen things that would freeze your blood... So dont threaten me, dont ever threaten me.
- Mind Rape: The Doctor, to the creature.
- Not So Different: Lucie, towards the end, muses whether the Doctor might be scarier than the Monsters.
- Sequel Hook:I came here with days to spare, and next time
- Shout-Out: At one point someone mentions "The entity would have gotten away" at which point Lucy quips
- Talking the Monster to Death:Ive seen entire species destroyed, civilizations left in ruins. Ive witnessed solar systems vanish in the twinkling of an eye. Ive seen things that would freeze your blood... So dont threaten me, dont ever threaten me. [...] My real fear is the things I don't enjoy. The things I've seen and never want to see again. [...] Now, entity, what can I show you... let's start with evil from the dawn of time, and go on from there, shall we? [...] I wouldn't make these things up — I couldn't! I saw them all! [...] Well, evil from the past is one thing. But I have seen the future, too... [...] Wouldn't you like to see what's coming? It scares the living daylights out of me. I wonder what it'll do to you. [...] Oh, no, wait-wait-wait! There's worse than that! There are the things that I am afraid I might do one day.