The Mercy tells the incredible true story of businessman and amateur sailor Donald Crowhurst (Firth) who entered the first non-stop, single-handed, round the world yacht race in 1968, hoping to publicise a new navigation device he had been working on in order to save his ailing business.

Leaving his wife, Clare (Weisz), and children behind he set off in his untested trimaran to circumnavigate the globe. However, due to his inexperience and the intense loneliness of the high seas, his journey soon turned into a dramatic and disastrous struggle for survival.