Skylark Three (1948) is the second book in the Skylark series and is set a year after the events of The Skylark of Space, during which year antagonist Marc "Blackie" DuQuesne has used the wealth obtained in the previous book to buy a controlling interest in the story's 'World Steel Corporation', a large company known for its ruthless attitude. When the story begins DuQuesne announces a long absence from Earth, to find another species more knowledgeable than the Osnomians allied with protagonist Richard Seaton. Shortly thereafter, DuQuesne and a henchman disappear from Earth. DuQuesne, by now aware of the 'Object Compass' trained on him, travels far enough to break the connection, then turns toward the 'Green System' of which Osnome is a part. Seaton discovers this, but is distracted by attempts to master a "zone of force": essentially a spherical, immaterial shield, with whose present form Seaton is dissatisfied for its opacity and impenetrability even by the user.
Seaton is then requested by his allies Dunark and Sitar, the crown prince and prince's consort of Osnome, to repel invasion by the natives of planet 'Urvan', Osnome's neighbor; whereupon Seaton and his millionaire sponsor, Martin Crane, accompanied by their wives and Crane's valet Shiro embark in the spaceship Skylark II to obtain the necessary minerals. Near the Green System, they are attacked by the hitherto-unseen natives of the planet 'Fenachrone', whose weaponry surpasses any known to Seaton or the Osnomians. Having used the 'zone of force' at first to conceal himself, and then to destroy the Fenachrone battleship, Seaton captures a leading crew-member, who reveals (upon interrogation) that the Fenachrone intend conquest of the entire Milky Way Galaxy, and eventually of the Universe, and that a message is already in progress toward the Fenachrone capital to summon aid. Discovering that Dunark and Sitar survived the destruction of their spaceship, the Skylark II tows the remnants of both vehicles to Osnome, where Seaton forces a peace treaty between Osnome and Urvan. Meanwhile, DuQuesne and his aide have also interrogated a Fenachrone and plan to capture an entire Fenachrone battleship for personal use.
Hoping to master the "zone of force", Seaton, Crane, and their wives travel from planet to planet of the 'Green System', in search of those already skilled in its use. Initially they encounter the Dasorians, an amphibious species of humanoid, who direct them to the Norlaminians, who possess full control of both matter and energy. A much larger successor ship, the Skylark Three, is built and equipped on planet Norlamin, where Seaton at first explores the Fenachrone's world, then orders the Fenachrone to abandon their conquest. Upon their refusal, Seaton destroys their reconnaissance spaceships, and then their home planet, killing all the Fenachrone except a colony led by one of their leading scientists. Equipped with a Fenachrone spaceship, DuQuesne also survives, but leads Seaton and the Fenachrone to presume his death. Increasing his technology still further, Seaton pursues and destroys the Fenachrone colony; but does not discover DuQuesne.

Question: Why are Seaton and Craine going to the Green System?
Answer: To the zone of force.