Doctor Who's Spooky Agatha Christie Tribute Also Tied Up A Long-Standing Plot Thread

The fun thing about being a time traveler is that you can often procrastinate on important tasks for as long as you'd like. This was part of the appeal for Amy (Karen Gillan) at the start of "Doctor Who" season 5; she was having second thoughts about marrying Rory the next day, so the Doctor's promise that she could travel through space and time for as long as she wants beforehand seemed like a great deal.

The rest of the season eventually dealt with Amy's doubts about Rory, but it once again ended with the characters procrastinating. This time, it was the Doctor who took his sweet time getting around to an important task: in the finale, he receives a phone call (seemingly from the future queen or king of England) regarding an emergency situation. In the future there's apparently a train that runs in space called the Orient Express, and there's a sentient mummy running around it, killing off the passengers one by one. 

Seems like urgent business, right? Well, the Doctor doesn't get around to dealing with it until season 8. By that point, the Doctor has a whole new face and a whole new companion, but as far as the person on the phone knows, he arrived straight away. 

The season 8 episode was a fun case of the show paying off a throwaway line long after casual fans remembered it. By the time "Mummy on the Orient Express" aired in 2014, the only people who could appreciate the joke were the hardcore fans who remembered those months on the fan forums after season 5's "The Big Bang." When speculating about what the annual Christmas special episode would be about, many fans assumed it would be an Agatha Christie-inspired romp, only to be disappointed when it went in a different direction. (Of course, the Dickens-inspired romp we got instead was so good that it was hard for fans to complain.) 

Another factor that made it easy to forgive the show's stalling on the Orient Express storyline is that, by the time of the season 5 finale, it hadn't even been that long since the show had last pulled off an Agatha Christie tribute episode. The season 4 story, "The Unicorn and the Wasp," was a murder mystery set in the 1920s that involved Christie herself (played here by Fenella Woolgar). 

The episode was based around "Crooked House," a novel that isn't as well known as "Murder on the Orient Express" but which nevertheless managed to get its own film adaptation in 1982, titled "Evil Under the Sun." The episode's main writer, Gareth Edwards, has talked about how he deliberately chose a comedic tone and that he and showrunner Russell T. Davies tried to squeeze in as many references to Christie's books as they could. The result was an episode that is rarely considered high-tier among Whovians, but which nevertheless has plenty of fans. 

It's a perfect episode to sit back and relax to, and in season 4 it serves as the final light-hearted story before things got dark. The next three episodes ("Silence in the Library," "Forest of the Dead," "Midnight") were all terrifying, and then the show was off to a very dramatic, high-stakes three-part season finale. "The Unicorn and the Wasp" was meant to be the last point where Donna and the Doctor's lives felt simple and carefree, and it definitely succeeds in that respect.

When "Doctor Who" did their Christie episode in season 8, with the 12th Doctor finally following up on that season 5 call, it was in a much darker period of the characters' lives. "Mummy on the Orient Express" was the first episode after "Kill the Moon," the episode where Clara and the Doctor have a big falling out. The episode is part of a period where Clara is very close to leaving the Doctor for good, and the Doctor is struggling more than ever with the question of whether or not he's a good man. (This is arguably the big thematic question of season 8.)

The result is that this episode is interesting, but hardly a laugh riot. Clara and the Doctor get into some serious arguments, and when the one-off characters die in this episode it genuinely stings. Here, writer Jamie Mathieson used Christie's general premise as an opportunity to explore the heavy, life-or-death questions the Doctor and Clara have been grappling with all season; the duo may end the story on a positive note, but they go on a difficult journey to reach that point. 

Overall, seasons 4 and 8 were dealing with two very different arcs for its main duos. Season 4 was the Tenth Doctor's final season, all about letting the audience enjoy him before he goes. Meanwhile, season 8 was all about Capaldi proving himself to the audience, convincing us (and Clara) to let him stick around. The result is a season 8 Christie tribute episode that was far spookier and more intense than the one before it. The storyline may have been introduced in season 5 as part of a throwaway joke, but the actual episode was a surprisingly grim affair.