I used to quite like this episode when I was young. Unfortunately, it really failed to hold up on rewatch, and I'm forced to come to the conclusion that this does a really poor job of executing on its premise. We'll start with the positives; Tennant and Tate's chemistry is absolutely on point. They're one of my top TARDIS teams to begin with, and this story gave them a lot to bounce off each other. It's quite funny, though some of the jokes felt rather forced at times. E.g., I laughed at the detox scene, but it was ultimately just a lot of padding that didn't go anywhere.

The fact that the Doctor had just been poisoned really fails to tie in with the rest of the story, and that's kind of the main issue I have with the whole thing; all the elements of it that shout "Christie", from the cyanide to the jewel thief to the cast of suspects are just window dressing. At no point is motive brought into the conversation. The alibis are pointless; everyone's lying and no one can corroborate anything. The jewel theft subplot is skimmed by and barely affects the plot at all. The plot gestures vaguely at the cast of suspects, a hastily assembled lot of caricatures pulled out of a Clue game* with no defining opinions or characteristics among them. This isn't a murder mystery; it's just mystery-themed. There's no drive to flesh out the plot any further because that would take away time that could be spent slobbering all over Agatha Christie as the Greatest Writer of All Time, much like Roberts' treatment of Shakespeare in s3.

Now. Am I being a bit unfair here? Perhaps. It would be difficult to squeeze a fully thought out Christie-style caper into the space of a New Who episode. On the other hand, it's difficult to say it couldn't at least have been done better. Roberts clearly bit off more than he could chew with this one; the weakest of s4 by a wide margin. If you want a proper murder mystery in Who, go watch The Robots of Death.

*literally! You've got the maid, the colonel, the young socialite, the wealthy older dame, the reverend, and the professor named after a fruit. I wasn't kidding about this being mystery-themed.