the three Nansalians--and, one by one, the Nansalians replaced them.

They had bleached their faces, and surgeons, working from photographs,
changed their features so that the three Nansalians were exact doubles
of the three astrogators. Then they acted. On three trips, one of the
men that went back as navigator was a Nansalian.

It was six years before they returned to Nansal, but when they finally
did, they had learned two things.

In the first place, the 'disease' which had killed Nansalians who had
come in contact with Satorians on Nansal was nothing but a poison which
acted on contact with the skin. The Nansalians who had gone to Sator had
simply been murdered. There was no disease; it had simply been a
Satorian plot to keep Nansalians from going to Sator.

The second thing they had learned was the secret of the Satorian
magnetic space drive.

It was common knowledge on Sator that their commander would soon lead
them across space to conquer Nansal and settle on a world of clear air
and cloudless skies, where they could see the stars of space at night.
They were waiting only until they could build up a larger fleet and
learned all they could from the Nansalians.

They attacked three years after the three Nansalian spies returned with
their information.

During those three years, Nansal had secretly succeeded in building up a
fleet of the magnetic ships, but it went down quickly before the vastly
greater fleet of the Satorians. Their magnetic rays were deadly, killing
everyone they struck. They could lift the iron-boned Nansalians high
into the air, then drop them hundreds of feet to their death.

The buildings, with their steel and iron frames, went down, crushing
hundreds of others. They practically depopulated the whole planet.

But the warnings of the three spies had been in time. They had enlarged
some of the great natural caverns and dug others out of solid rock. Here
they had built laboratories, factories, and dwelling places far
underground, where the Satorians could never find them.

Enough men reached the caverns before the disaster struck to carry on.
They had been chosen from the strongest, healthiest, and most
intelligent that Nansal had. They lived there for over a century, while
the planet was overrun by the conquerors and the cities were rebuilt by
the Satorians.

During this century, the magnetic ray shield was developed by the hidden
Nansalians. Daring at last to face their conquerors, t