2078, making sure that your records
would stand up under all but the closest examination. From then on,
when Martin carried you out from the fire, you stored your own memory
of that time-area and became a legitimate member of that society."

"But how could we pose as David's _parents_, if he was my father?"

Ann smiled. "Both David's parents were killed in the New York bombing;
we knew that David survived, and we knew where he could be found.
There was a close physical resemblance between you and the boy,
though actually the resemblance was backwards, and he accepted you as
a foster-father without question. With you equipped with a complete
memory of your marriage to me in that time, of David's birth, and of
your own history before and after the bombing of New York, you fit in
well and played the part to perfection. Also, you acted as a control,
to guide us, since you had no conscious knowledge beyond that
time-area. Martin and Morrel were to be the assassins, the Intruders,
and I was to keep tabs on you--"

"And the success of the attempt?"

Ann's face fell. "We don't know yet. We don't know what we
accomplished, whether we stemmed the war or not--"

The tall man who had stepped into the room moved forward and threw a
sheaf of papers on the floor, his face heavy with anger, his voice
hoarse. "Yes, I'm afraid we do know," he said bitterly.

Martin Drengo whirled on him, his face white. "What do you mean,
Markson?"

The tall man sank down in a chair tiredly. "We've lost, Martin. We
don't need these calculations to tell. The word was just broadcast on
the telecast. Farrel Strang's armada has just begun its attack on
Mars--"

* * * * *

For a moment the distant bombing was the only sound in the room. Then
Martin Drengo said, "So he gave the order. And we've lost."

"We only had a theory to work on," said Morrel, staring gloomily at
the curved window. "A theory and an equation. The theory said that a
man returning through time could alter the social and technological
trends of the people and times to which he returned, in order to
change history that was already past. The theory said that if we could
turn the social patterns and technological trends just slightly away
from what they were, we could alter the entire makeup of society in
our own time. And the Equation was the tool, the final check on any
change. The Equation which evaluates the sum of social, psychological
and energy