South Wales, around a year before the outbreak of World War Two. David Goliath is an unemployed African-American ship's stoker (who once worked down a mine for five years in the United States) and made his way to Cardiff three months earlier.

In search of work, he travels into the Welsh countryside. On his journey, David befriends a traveller named Bert, who tells him about a coloured man named Blackie Ellis, who used to work in the Glen Colliery. This information reassures David that, even in the Depression, not all collieries operate a 'colour bar'.

Arriving in the mining village of Blaendy, David and Bert busk for money in the street, where they are overheard by Blaendy's choirmaster, Dick Parry. Impressed with David's singing, Dick takes him home to meet his wife. David becomes their lodger, and Dick finds him a job in the local colliery as well as a place in their choir.

The choir needs a bass for its planned performance of Elijah for the forthcoming eisteddfod. But not everyone is impressed with the newcomer. Seth Jones, one of the miners, objects to David's colour and resents his employment. Dick's response is to support his new friend, while the rest of the Blaendy community ignore Seth and welcome David.

Settling in with the Parry family, David looks forward to performing with the Blaendy Male-Voice Choir at the coming eisteddfod, but tragedy lies ahead. On the day of the eisteddfod, Dick is killed in a pit disaster. At the eisteddfod, which has been postponed for a month, David pays tribute to his pal with an emotional rendition of the spiritual 'Deep River'.

After the eisteddfod, unemployment takes its toll on the villagers, who suffer great hardship. David joins the community in their fight against the closure of their colliery. Happily, the mine is reopened to help with the war effort, but tragedy strikes again when David sacrifices himself to save Dick's son, Emlyn, in another pit disaster.