Doctor Who's latest episode, "Kerblam!", made references to the show's David Tennant and Matt Smith eras that proved to be much more than simple moments of fan service. In this week's adventure, Team TARDIS were whisked to a future where humans struggled to find work due to a widespread preference for robotic employees. Tipped off by an anonymous plea for help, the Doctor and her companions seek the message's source at Kerblam!, the largest retailer in the galaxy, proud to hire a minimum quota of 10% organic workers.

The episode's final act manages to subvert the expectations of both the Doctor and of viewers by revealing that the request for help didn't come from a human employee, but from Kerblam!'s own computer system. The system had discovered that a political activist had infiltrated the company and was planning on committing an act of mass murder to try and break society's trust in robotic employees. Needless to say, the Doctor soon put a stop to that.

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With an intriguing mystery, creepy villains and sharp social commentary, "Kerblam!" was largely praised on social media, but fans were also excited by several callbacks to modern Doctor Who history. These references, however, weren't chosen at random and actually held a far deeper significance than first appeared.

Matt Smith's Fez Was A Big Plot Clue

The first of these references came very early in the episode, with the Doctor receiving a parcel from Kerblam! containing both the "HELP" message that kicks off the story and a Fez that she claims to not remember ordering. The Fez is clearly a reference to Matt Smith's Eleventh Doctor who famously wore this style of hat in the Doctor Who season 5 finale, "The Big Bang", insisting to those nearby that Fezzes were cool, even if Amy, Rory and River Song failed to share his enthusiasm.

It's certainly possible that the Eleventh Doctor actually did order this item and the Kerblam Man simply delivered it to the TARDIS at the wrong point in time. After all, regenerating time-travelers must be a mailman's worst nightmare. However, the finale of the episode reveals that the Kerblam! computer system consciously sent a request for help to the Doctor. While the AI could have inserted this message into a package the Doctor did genuinely order, it's perhaps more likely that the system picked out and sent the Fez of its own accord as a way of grabbing the Doctor's attention. As with many modern retailers, it might even have selected the item based on previous purchases.

David Tennant's Agatha Christie Was A Big Easter Egg

After Yaz speculated that the Doctor was probably the type of child who'd poke a stick into a wasp's nest, the Gallifreyan responds with the line "talking about wasps, did I ever tell you about me and Agatha Christie?" -  a direct callback to Tenth Doctor episode, "The Unicorn and the Wasp". This fun offering saw the Doctor and Donna team up with the iconic British author to deal with a murderous alien that had taken on the form of a giant wasp.

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At face value, this is just another of the Doctor's many offhand comments that reference previous episodes and maybe this is exactly how writer Pete McTighe intended the line. However, Agatha Christie actually has a deeper connection to the Doctor's history with deadly robots as her work inspired the Fourth Doctor story, "The Robots of Death". This classic adventure saw a workforce of robots originally designed to assist their human counterparts get reprogrammed and begin a killing spree of anything organic.

Clearly, there are a number of parallels between "Kerblam!" and "The Robots of Death" and while the Agatha Christie connection may have been a complete coincidence, it's certainly a very apt connection to make given the similar themes covered in these two stories.

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Nothing excites fans quite like subtly referencing a previous episode and, given its lengthy history, Doctor Who does this better than most. However, these two instances arguably came at just the right time. While Jodie Whittaker's debut season has been met with a mostly positive reaction, it has also represented a fairly significant departure for the series and there hasn't been too much to directly connect the Thirteenth Doctor to her past (other than the use of Venusian Aikido). As such, these references were not only smarter than they first seemed, but also served to anchor the new Doctor to two previous regenerations.

Next: Doctor Who Season 11 Is Rewriting The Doctor's Time Lord Backstory

Doctor Who season 11 continues with "The Witchfinders" November 25th on BBC.