Third appearance of Guardians of the Galaxy.
First appearance of Korvac.
Origin of Nighthawk.
While investigating a temporal disruption, Dr. Strange, Valkyrie, and Hulk find Eelar, a creature mutated by the alien Badoon to destroy humanity. The group end up meeting the Guardians of the Galaxy, whose ship the Captain America had crash landed in the 20th century.
As the Guardians are transported to the location of the Defender's battle against Eelar, Vance Astro's younger self -- Vance Astrovik -- finds the Guardian's ship and meets with Martinex.
Defeating Eelar, and reversing it's mutation, the Guardians explain that they are in the modern era to stop the Badoon invasion. During an exchange of information, the Defenders agree to return to the Guardian's 31st century and help them defeat their Badoon oppressors.