In the latest episode of Doctor Who, Jodie Whittaker's 13th Doctor takes her companions back in time to 1947, right at the historic moment when British colonial rule of India came to an end in an event known as the Partition of India. Beyond the political significance, this time travel trip is deeply personal for Yasmin Khan (Mandip Gill), because her grandmother is smack in the middle of these world-changing events.
In this way, "Demons of the Punjab" will remind Who fans of the 2005 episode "Father's Day," in which the 9th Doctor took Rose Tyler to see her dad in 1987. But "Demons of the Punjab" is not "Grandmother's Day" for the new season of Who, and that's because despite having a number of elements in common with "Father's Day," it flips the script on that story in one hugely significant way.
In the canon of Doctor Who, the Doctor has a bad track record with things called "fixed points in time." What does that mean? The long answer involves paradoxes and how the Time Lords of Gallifrey used to regulate the flow of time in the days before the Time War with the Daleks. The short, real-world answer is that the show's writers have often needed to prevent the Doctor from changing events in history, for obvious reasons. In this case, the 13th Doctor knows she can't stop the Partition of India from happening, nor can she prevent the death of a man who isn't Yaz's grandfather but nearly could be.
In "The Fires of Pompeii" the 10th Doctor told Donna Noble "some points are fixed," meaning he could not change history and save everyone living in Pompeii from the eruption of Vesuvius. He called this "the burden of the Time Lords." In "Demons of the Punjab" the aliens from the Thijarian hive make a direct reference to this concept when they tell the Doctor that "the fixed force of time cannot be stopped." She nods knowingly, and that's because she's messed up fixed points in time before.
In "The Waters of Mars," the 10th Doctor changed a fixed point in time, which caused the TARDIS to freak out and for the people affected by the change to turn on him. In "The Wedding of River Song," the 11th Doctor violating a fixed point in time created a fractured and freaky version of the universe, where multiple timelines are happening all at once, resulting in Charles Dickens appearing on TV and pterodactyls attacking children in public parks.
As a result, the Doctor finally knows that screwing with these supposed "fixed points" can cause huge headaches, which probably explains why she accepts the death of Prem (Shane Zaza), the doomed first husband of Yaz's grandmother, Umbreen (Amita Suman and Leena Dhingra).
But "Demons of the Punjab" subtlely references the canon of Doctor Who in another way, too. In the final Peter Capaldi episode, "Twice Upon a Time," the all-powerful alien artificial intelligence called "Testimony" is revealed in the end to be a benevolent historian, harvesting people's memories before they die for enteral posterity. In that episode, the 12th Doctor is elated, saying "it's not an evil plot!"
This is pretty much identical to the Thijarian hive, the titular demons in "Demons of the Punjab." The episode begins with the audience thinking they are assassins, but as it goes on, we realize they are just interstellar mourners, monsters who are not monsters. This last twist is also connected to how "Demons of the Punjab" differs from "Father's Day," despite having nearly an identical structure.
Both "Father's Day" and "Demons of the Punjab" are bookended with one of the Doctor's companions talking to a relative about events from the past and listening to old family stories. In "Father's Day" it's the child version of Rose talking to her mom, and in "Demons of the Punjab" it's Yaz talking to her grandmother. Both the 9th and the 13th Doctors are hesitant and concerned about taking their companions back in time but do it anyway. Both episodes deal with the inevitable death of a family member, though in Yaz's case it was one she never knew she had.
And, most relevantly, both feature alien monsters who appear to (apparently) wreak havoc.
In "Father's Day," when Rose saves her dad for getting hit by a car, giant winged creatures appear called Reapers. The 9th Doctor says they are there to "sterilize a wound in time." In "Demons of the Punjab," the monsters are similarly concerned with not messing up time, but decidedly less aggressive. In both cases, the monsters in these episodes are motivated by preservation, not destruction, despite looking and acting pretty destructive. The Thijarian hive is clearly the ones you'd rather hang out with, but in terms of big themes and plot structure, they have similar purposes.
The biggest difference then is the ending. In "Father's Day," Rose recognizes her father has to die, and yet, she actually changes history by being with him in the moment of his death. We know she changes history because the closing scene with her and her mom reflects a different conversation. Now her mom remembers a grown woman holding Pete Tyler as he died.
However, "Demons of the Punjab" resists this kind of thing, because the Doctor, Yaz and co. don't rewrite history at all. When Yaz and her grandmother chat in the final scenes, it's totally clear that Umbreen's memories of the past have not been changed by time travel. She doesn't suddenly recognize Yaz as being both a mysterious stranger in her past and her granddaughter in the present. Nothing has changed at all, in fact. The Doctor and her companions turn their backs as Prem is shot, and time marches forward. This kind of thing happened earlier in the season in "Rosa," when the Doctor and her companions became part of history, rather than changing it.
So, for all of its similarities to Doctor Who episodes of the past, "Demons of the Punjab" (along with "Rosa") might represent something special about the 13th Doctor: she's actually pretty responsible with time travel. Even when it hurts.