They were bursting out onto the roof. Frog-guards were on it who came in a hopping rush toward them, force-pistols raised. But a giant leap took Hackett among them, to amaze them for a moment with great flailing blows. Sarja had leaped for the nearest flying-boat resting on the roof, and was calling in a frantic voice to Norman and Hackett. Norman was turning toward Hackett, the center of a wild combat, but the latter emerged from it for a brief second to motion him frantically back. "No use, Norman--get away--get away!" he cried hoarsely, frenziedly. "Hackett--for God's sake--!" Norman half-leaped to the other, but an arm caught him, pulled him desperately onto the boat's surface. It was Sarja, the long craft flying over the roof beneath his control. "They come!" he panted. "Too late now--" Frog-men were pouring up onto the roof from below. Sarja sent the craft rocketing upward, as Hackett gestured them away for a last frantic time before going down beneath the frog-men's onslaught. * * * * * The roof and the combat on it dropped back and beneath them like a stone as their craft ripped across the silvery dusk over the mighty frog-city. They were shooting toward the city's center, toward the green pool that was the entrance to the water-tunnel, while behind and beneath an increasing clamor of alarm spread swiftly. Norman raged futilely. "Hackett--Hackett! We can't leave him--" "Too late!" Sarja cried. "We cannot help him but only be captured again. We escape now and come back--come back--" The truth of it pierced Norman's brain even in the wild moment. Hackett had fought and held back the frog-guards only that they might escape. He shouted suddenly. "Sarja--the water-tunnel!" A half-dozen boats with frog-guards on them were rising round it in answer to the alarm! "The force-gun!" cried the green man. "Beside you--!" Norman whirled, glimpsed the long tube on its swivel beside him, trained it on the boats rising ahead as they rocketed nearer. He fumbled frantically at a catch at the gun's rear, then felt a stream of shells flicking out of it. Two of the boats ahead vanished as the shells released their annihilating force, another sagged and fell. From the remaining three invisible force-shells flicked around them, but in an instant Sarja had whirled the boat through them and down into the water-tunnel! Norman clung desperately to his seat as the boat flashed down through the waters, and then, as Sarja sent it flying out through the great tunnel's waters, glimpsed, close behind, the beams of the three Rala boats as they pursued them through the tunnel, overtaking them. Could the force-shells be fired under water? Norman did not know, but desperately he swung the force-gun back as they rushed through the waters, and pressed the catch. An instant later beams and boats behind them in the tunnel vanished.