Yaz's grandmother in enigmatic about her past at her birthday celebration so Yaz convinces The Doctor to take them to India in the 1950s so she can see her grandmother as a young woman. Using the watch her grandmother gave her, the T.A.R.D.I.S. delivers them to 1947 instead, the day that India is partitioned to create Pakistan. Yaz's grandmother, Umbreen, is a Muslim woman and she is about to marry a Hindu neighbor named Prem (who is NOT Yaz's grandfather). Umbreen's mother thinks the marriage is cursed and Prem's militant younger brother, Manish, is against the marriage altogether.
The Doctor detects an alien presence and she, Prem, and Ryan discover a Thijarian hive ship after seeing them standing over the dead body of the holy man who was going to perform the wedding ceremony. The Thijarians are known assassins. The Doctor takes a capsule of mysterious powder from the ship and creates a safe perimeter around the farm to protect them from the Thijarians temporarily.
Umbreen is determined to get married but the Thijarians abduct The Doctor first and explain that they are no longer assassins. Their mission now is to bear witness to un-commemorated deaths. The Doctor returns to the Companions with the knowledge that Prem will die the next day. The wedding proceeds but Manish brings a local militia to the farm to drive Umbreen and her mother out. They kill Prem when he tries to intervene and bar them from the farm. It was Manish who killed the holy man. Prem's essence is stored by the Thijarians with their memorial to the unacknowledged dead.
Returning home, Yaz is content to let her grandmother have her secrets.