After a family tragedy, a young American couple joins some friends at a midsummer festival in a remote Swedish village. But, what begins as a carefree summer holiday takes a sinister turn when the insular villagers invite their guests to partake in festivities that grow increasingly unnerving and viscerally disturbing.

It's the long hot summer of 1959 and wedding season is in full swing in the Cambridgeshire village of Grantchester. As the Reverend Will Davenport unites happy couples in holy matrimony, Detective Inspector Geordie Keating finds himself trying to repair his own marriage to estranged wife Cathy.