From The Heavens/Re-summoned

A familiar, bald head materialized. It was broad-faced, and held a heavy brow over pupil less eyes. It was Uatu, the Watcher. "Pardon my interruption, Guardians," Uatu said. "But the business we had begun merely 60-Earth-days ago has not concluded. Galactus has disappeared. I need not remind you, the very existence of the universe depends upon his continued well-being. Coincidentally, the Asgardians soon face Ragnarok, the final cataclysmic battle to end their world. This is most strange, given that they have defeated the forces prophecies predicted will devour them." his thin body materialized beneath his head, as did the form of Silver Surfer from behind him.

"I fear the 2 events are related, for I sense a change in the nature of the universe. My memories of my race have grown faint, and i find my awareness of the universe dimming. I am sure that the bonds of reality are also weakening. You, too, will soon begin to feel a disconnect from all-that-is. If our universe is to survive, you must prevent Ragnarok and return Galactus to our dimension."

-

In order to travel to Asgard, the Guardians visit Earth and specifically the Sanctum Sanctorum of Dr. Strange. Here they meet with the good Doctor to discuss the dimensionally displaced Asgardian, Angela, whom they had met prior to the events in Attilan.

Dr. Strange explains the origins of Angela. "Long ago, in a time before time, in a time before history when men had not learned speech and hunted great beasts of the land, there was a 10th-Realm called Heven in which a race of beings called Angels lived. It was ruled by an entity called the Queen of Angels. During this prehistoric time, humans were constantly harassed by the Asgardians. The queen made a deal with Odin, ruler of Asgard, to protect the weak earthlings from his own subjects. In return, Odin would pay the Angels in full. The Angels of Heven revered payment above all else. But, the greedy queen also made a deal with the inhabitants of Jotunheim to form a strategy capable of defeating Asgard, which rightfully angered the vengeful Odin. The queen then offered Odin another deal, one that would lead to her ultimate betrayal of the giants in Jotunheim. Paying off the Angels to betray the giants, Odinn realized that their queen had no honor. Valuing payment above all, and viewing Odinn's ideas of honor as swindle, and insult the Queen of Angels instigated war against Asgard. Soon after, Heven realized it was outmatched against the might of the Asgardians. Realizing t she could not win, the queen of Angels abducted and threatened to kill the first-born child of the pairing between Odin and Freyja, Aldriff Odinnsdottir. She used the child as blackmail against Odin, expecting the surrender of Asgard as her payment. Odin refused to pay, and the Queen seemingly killed his infant daughter. Odin's inevitable anger led to the severing of Heven from the 9 realms and its isolation in a sealed pocket dimension. Unknown to Odin, however, his child had survived. The queen had ordered her personal handmaiden to get rid of the baby, but the maiden nursed and cared for the child, raising it as her own as Angela. Now, centuries later, when the Elders attempted to transport Ego through space as a diversion, they tore minute rips i to the very fabric of space and time itself. Through one of these rips Angela had accidentally been transported from Heven into Midgard (the Earth dimension)."

Because of the opening of the doorways between dimensions, Angela would be able to use her Asgardian heritage to open the gateway to Bifrost the rainbow bridge into Asgard. So, Dr. Strange, and Angela join Drax, Gamora, Rocket Raccoon, Groot, Mantis, and Star-Lord as they embark to Asgard.