• Jodie Whitaker (The Doctor)
• Bradley Walsh (Graham O'Brien)
• Mandip Gill (Yasmin Khan)
• Tosin Cole (Ryan Sinclair)
• Charlotte Ritchie (Lin)
• Nikesh Patel (Mitch)
• Daniel Adegboyega (Aaron)
• Darryl Clark (Will)
• Connor Calland (Richard)
• James Lewis (Farmer Dinkle)
• Laura Evelyn (Polly)
• Sophie Duvall (mum)
• Callum McDonald (1st teen)
• Harry Vallance (2nd teen)
• Michael Ballard (sergeant)

A long time ago, on the battlefields of Britain, an army of enemies cam together to face an impossible opponent. This army only just won the battle: the remorselessness of their enemy was so great and the army's fear was so absolute the survivors made a pact. Cutting the body into three pieces and taking them to opposite ends of the world, they vowed to guard the burial sites forever more. They swore an oath of secrecy and to pass the guardianship of the three sites over generations. Except one of the custodians was kiled before completing his mission, his body left to be uncovered centuries later by two people working on New Year's Day on an archaeological dig.

Mitch and Lin discuss personal matters until they find something beneath the hand of the skeleton. Bagging it, the artefact is left on a table with other artefacts, under a UV light. At the same time, disturbances at the two burial sites result in the artefacts there disappearing, somehow uniting with the third artefact.

As the Doctor, Graham, Ryan and Yaz visit 19 New Year celebrations throughout time and space and get ready to round it up to a 20th, the TARDIS alerts them to a bizarre spatial shift. An ancient enemy is about to be reborn.