Gamora explains

A short while later, after they'd hauled the **** Thor and Hulk aboard, the Guardians and the barely clothed Valkyrie gathered around as Gamora began to speak.

"Thanos is... a sick creature." Gamora said. "He's insanely powerful, but uses that power to **** an humiliate others who think themselves strong. Especially women. His purpose is to see women brought low."

"My sister Nebula and I were his enforcers. We traveled the cosmos, conquering worlds, making all the women surrender their clothes or face extinction. Thanos... he demanded we be naked the whole time as we did his will."

"Eww, isn't he your father?" Rocket asked.

"Not really." Gamora sighed. "He **** us from our homeworlds in exchange for their obedience. We were his weapons, and his amusement. Nebula had it worse then me. She hated being naked, and Thanos's mockery made it no easier. Eventually he had cybernetic attachments put on Nebula so he could control her better and make her act to his amusement."

"That's all... interesting, but what does he want?" Valykrie said, still tugging the undersized shirt over her crotch.

"Thanos has always wanted one thing: the Infinity Stones." Gamora explained. "With all of them he can rewrite reality to his will. I shudder to think what sort of horrifying, twisted reality we will be left in if he gathers them all. He already has one, from conquering Xandar."

"Two." Valkyrie explained. "There was one on our ship, though we did not know."

"The question is, where are the rest?" Quill asked.

"Earth." Everyone turned to see Thor staggering to his feet. "The rest of the stones... are on earth."