the doctor persuades the deciders to give him equipment to examine the cells of the marshspider and marshchild and deduces that they are from identical dna sources. the situation is complicated, however, when a transformed romana releases the emergency exits and allows the marshmen to invade the starliner. the colonists retreat before the creatures, many of whom are more inquisitive than dangerous. nefred is mortally wounded while fleeing; his last admission is that the colonists cannot return to terradon, because they have never been there. it is realised that the present-day alzarians are actually a subspecies of the marshmen, who wiped out the starliner's original terradonian crew and then gradually evolved into human form to take their place.