Scientists go missing on 20th Century Earth and the Third Doctor deduces they are being kidnapped through time. Arriving in the middle ages with journalist Sarah Jane Smith he discovers a lone Sontaran warrior, commander Linx, who has crash-landed and is stranded.

Irongron, a robber baron and leader of a local bandit gang, is offered 'magic weapons' in exchange for shelter, so the ship can be repaired. The two strike an uneasy bargain.