I remember once reading an Agatha Christie interview where she discussed the rumor that she wrote backwards and only adding the elements that allowed the mystery to be solved after the rest of the book was written. That's what Phobos feels like. The twist of the fear entity  feels artificial and it doesn't feel integrated with the plot naturally. There's very little build-up other than a single scene or two of Kai trying to cause fear. the fear entity is a great concept, its Lovecraftian in nature, and it has the power to feel threatening - if it were explored properly.  Yet only introducing it in the last 10 or so minutes meant it couldn't be explored. And the 'rules' governing its behavior felt bizarre (I'd give them a pass if the rest of the story were better) and Kai's goals seem convoluted  - surely there were better more efficient ways of making people feel afraid? Furthermore, the way the Doctor defeats the monster feels lazy.

Then there's the side plot about Farl and Amy. This was just fine. It didn't really feel resolved though. Their family drama felt unexplored and unresolved with no ending. Also, the racism directed to Farl feels like a poor plot point considering he seemingly conforms to the expected stereotype of his species. There's also Hayd and Drew. Drew's queerness is just lazy. Its thrown into the script with no build-up and no resolution. I get 2006 was a different time but it still feels pointless and unexplored (like many elements of this story).

Overall, I can picture a world where Phobos was a much better story about fear, where it explored its characters and their fears. Farl and his families prejudice, Kai and his fear of the entity as well as losing Eris, Amy and raising a child alone, Drew and his love for Hayd. Instead with have a story that is serviceable but feels rushed and like its missing solid 10 minutes.

Also the fact Amy just accepts Lucie is a time traveler feels absurd. I get this is set half a century in the future, but that still feels like something that's just silly. Focusing on Lucie in this story, she doesn't feel like herself. She feels poorly characterized. My best example is the joke about the House of Lords. That's not something I can see her saying. It feels out of character, like the author thought of it and without wondering if it was something she would say.

I will praise it for being fine. The dialogue feels relatively natural for most of the characters, its not drawn-out, it does show more of the Doctor and Lucie becoming friends. But, just because it is serviceable doesn't mean its good.