With only a single, vulnerable mole available for embarking on deep-draft ships, the Royal Navy requisitions civilian vessels in Great Britain that can get to the beach. In Weymouth, a civilian sailor Dawson, with his son Peter, sets out on his boat "Moonstone", rather than let the Navy commandeer her. Impulsively, their teenage hand George joins them, hoping to do something noteworthy. At sea, they rescue a shivering shell-shocked soldier from a wrecked ship. When he realises that Dawson is sailing for Dunkirk, the soldier demands that they turn back and tries to wrest control of the boat; in the scuffle, George suffers a head injury that renders him blind. Elsewhere, three RAF Spitfires cross the English Channel, heading towards Dunkirk, tasked with defending the evacuation. After their leader is shot down in a dogfight, one of the pilots, Farrier, assumes command, but the other Spitfire is hit and ditches. Its pilot, Collins, is rescued by "Moonstone".