Sturdy, undistinguished sea melodrama--featuring the British submarine Proteus, which is on the first trial run of its top-secret ""Sea Leopard"" anti-sonar device. As the most stringent test possible, it has been sent into the Barents Sea off the Norwegian coast, an area lined with a set of Soviet sonar devices. But the Soviets are aware of ""Sea Leopard"" and have devised a trap for Proteus: they let a sub lie doggo and send out distress signals which attract the Proteus, then box in the British sub, damaging and grounding it with heat-seeking torpedos; furthermore, naval superman Valery Ardenyev, underwater operations specialist, enters the escape hatch in the torpedo room, knocks out the crew with gas in the air-condioning system, and the sub is floated by airbags to a Soviet drydock. Meanwhile SIS Deputy Director Aubrey has his agent Patrick Hyde scouring the English countryside for Quin--the ""Sea Leopard"" inventor who knows how to repair the device. (Hyde must track down Quin's rock-groupie daughter Tricia, who's an unwilling collaborator.) And the third sphere of action is led by Ethan Clark, US naval liaison to the Admiralty, who is flown by Aubrey to the countryside near the captured sub: eventually he'll repair the damaged device, on radioed instructions from Quin, and the Proteus--once more rendered invisible to detection--bolts out of the drydock through a mined Soviet net to safety in the Barents--with, however, Ardenyev still on board. . . . Technological fun for sonar specialists and navy readers; but there's too much padding as Hyde chases Tricia, and neither the suspense nor the characterization is on the level of Thomas' earlier, better work.