Yaz's family is celebrating her grandmother, Nani Umbreen's birthday. Nani gives Yaz's mom some letters, a pressed flower to Sonya, and a watch with a shattered face to Yaz. When Yaz's father offers to have the watch fixed, Nani emphatically states that the watch must never be fixed and then refuses to speak of it anymore. Nani states she was the first woman married in Pakistan as well as the first Muslim woman to work in a South Yorkshire textile mill. However, despite the many stories she has in her memory, she states that Yaz needs to be older before she can hear them.
Yaz asks The Doctor to take her back in time so that she can see her grandmother in her youth. After a bit of hesitation, The Doctor agrees to take them for an hour, just so Yaz can see her grandmother from a distance.
They arrive but it isn't the city of Lahore in the 1950s. Instead, they're in Northern Punjab. As The Doctor tries to scan the region with her sonic, she and the Companions feel a sudden psychic attack. A wagon arrives, driven by Prem, and he warns them that the roads aren't safe these days. Yaz asks about a woman named Umbreen and he offers them a ride.
A holy man, a Sadhu, named Bakti asks Prem if he's running transport now. Prem offers him a ride but he prefers to walk and promises that he'll be there. Prem's apparently running late and it's going to upset Umbreen. As they ride off, something in the treetops is armored and watching them.
They arrive at Prem's home and it turns out he's marrying Umbreen the next day. Yaz is confused because Prem isn't her grandfather although he is wearing the watch Nani gave her. Umbreen's mother calls everyone into the house for a big announcement. His younger brother, Manish, tells them he has advanced knowledge and runs behind the house.
Graham notes that the hour's up but Yaz won't leave now because she has more questions than answers.
Manish asks for help and Yaz goes while The Doctor, Graham, and Ryan go inside to hear the announcement on the radio of the partition of India. The travelers have arrived on August 17th, 1947, the day British India was divided into India and Pakistan.
Manish had staked a flag signifier into the stream behind the house, delineating the Indian side from the Pakistani side. He believes all Muslims should move into Pakistan. Prem is conflicted and Umbreen is outraged. At that moment, The Doctor experiences another psychic wave and two demons appear nearby. Umbreen's mother declares the days are cursed and The Doctor tries to take scans of the entities before they disappear.
They do disappear and The Doctor is determined to find out what they are and what they're up to.
They run into the wooded area and find the beings standing over the holy man, Bakti. The beings tell them to stay away and creating a screeching noise that knocks the travelers to their knees. Prem, coming up behind them with a rifle, takes a shot at them and they disappear again.
The Doctor asks Prem why he wasn't surprised by the demon appearance. He says that he's seen them before, during the war he fought it. He asks them who they actually are, not believing that they're Prem's family anymore. The Doctor's sonic detects clues of an alien engine nearby. Prem insists on coming with them on the demon hunt.
After warning Yaz to be careful what she says to her grandmother, The Doctor heads into the forest with Prem and Ryan. They find a transmat doorway that drops them into a buried spaceship. The Doctor gets her sonic to activate the computer and discovers that the spaceship is actually a Thijarian Hive. Thijarians are the deadliest assassins in the universe. Accessing the data drive again, The Doctor discovers that Bakti was their target.
The Doctor and Ryan press Prem on where he'd seen the demons before and he admits that they killed his older brother on the battlefield in Singapore. The data drive console ejects a capsule containing powder like they found on Bakti's body. When The Doctor takes it off its base, it triggers an alarm and the Thijarians arrive. The Doctor warns them off and uses the sonic to reverse the transmat to send them back into the forest. When she yells for Ryan and Prem to get out of the forest, they disappear again. She discovers a mini transmat lock on the tree and confiscates it, bringing Prem and Ryan back. The Doctor runs ahead, collecting mini transmat locks as she goes.
Back at the farm, Yaz and Graham try to explain Bakti's death. Manish is upset and sets out to retrieve the body. Umbreen's mother, Hasna, restates that the marriage is cursed but offers to help prepare the body for burial. When they've left, Yaz asks Umbreen if she still plans on marrying Prem after this. Umbreen states that in a world of uncertainty, Prem is the only thing she is sure of.
Yaz and Graham have a talk about things and then they hear Prem and Ryan yelling they've lost The Doctor as they run towards the farm. The Doctor comes up from behind and tells them to get into the barn. She plants and reprograms the mini transmat locks in the field. When the Thijarians appear in the barn, she activates the locks she's set up and exiles them off the farm, hopefully for a few hours. Umbreen is determined to celebrate and be married in the morning.
The Doctor opens the Thijarian capsule and scans it with the sonic, causing it to spark violently, overloading it. She lists things she needs to test the powder manually and the team spreads out to get them.
In the evening, The Doctor and Yaz join Umbreen and Hasna for the ritual henna hand tattooing. Umbreen and Hasna get into an argument about the marriage. Umbreen asks The Doctor to officiate the wedding.
Ryan and Graham are playing cards with Manish and Prem. Manish and Prem get into an argument about him getting married to a Muslim woman when the country's been partitioned. Manish storms out.
Back in the barn, The Doctor discovers that the capsule powder is the densest organic material she's ever encountered. The transmat locks start to shake as the Trijarians break through and reappear, grabbing The Doctor, and disappearing again.
The Trijarians explain to The Doctor that they are no longer assassins, that they now travel time and space, bearing witness to deaths that occur alone and without family. They came to honor Bakti's passing and to honor Prem's as well which will happen the next day.
The Doctor is returned to the barn and tells the Companions that Prem will die but they can't let anyone know. Manish disappears early the next day but returns just as The Doctor officiates the wedding and Yaz and Hasna help tie the bride and groom's hands together in a Hindu tradition.
Umbreen tries to heal the rift by toasting Manish at the wedding feast in the barn but is rebuffed. The Doctor follows him, prompting Prem for his speech before she leaves. Prem offers a "mahr" to Umbreen in the Muslim tradition, his wristwatch, which he drops in his nervousness, causing the face to shatter. She accepts it lovingly.
The Doctor follows Manish into the house where he gets Prem's rifle. He threatens her when she asks him about killing Bakti but walks out instead when they hear approaching horses. The Doctor runs to the barn to warn Prem, Umbreen, and Hasna. Umbreen and Hasna run home to gather things they want to take with them. Manish goes out to meet the approaching men and leads them back to the farm. Prem sends Umbreen and Hasna to the forest while he tries to talk to his brother.
Prem confronts Manish and the militia in the field. The Thijarians appear and promise to watch over him in his final moments. The Doctor and the Companions walk away and hear the gunshot that kills him. The Thijarians return to their hive, bringing Prem's essence and add his face to their memorial.
In the T.A.R.D.I.S. Yaz asks if Umbreen made it to Lahore okay and The Doctor confirms that she did. Back in Sheffield, Nani notices Yaz's henna and says it's a terrible design. She offers to tell Yaz the story of the watch but Yaz says to save it for another day.