During WW II, the US navy used the island in Southern Pacific to build a military base and when the islanders met the American soldiers it was a tremendous cultural shock.
The islanders have believed that the Americans were demi-gods. During this period, John Frum, an American pilot landed on Tanna. No one knows him except the islanders.
They believe that John Frum is their prophet and they still wait for his return. Their customs have been shaped by this belief.
"...they (cargo cults) do provide a fascinating contemporary model for the way religions spring up from almost nothing. In particular, they suggest four lessons about the origin of religions generally" - as Richard Dawkins stated in The God Delusion, the documentary shows us the evolution of a religion.
Except that, this is a sad story of pride, loneliness and hopelessness.