The Thanos Quest
Thanos looks into the Infinity Well to gain knowledge on how to best serve Death’s wishes
Thanos explains to Death about the 6 Soul Gems and gains her permission to set out and obtain them
Thanos sets out to find the Elders of the Universe who each possess a gem:
From the In-Betweener, who is incarcerated in the Nexus of All Realities by Lord Chaos and Master Order, Thanos gains the Soul Gem which formerly belonged to Adam Warlock
From the Champion he gains the Power Gem and strands him on Earth after he destroys his own planet
From the Gardener he gains the Time Gem
From the Runner he gains the Space Gem after manipulating the Time Gem to age him
From the Collector he gains the Reality Gem after trading it for the Runner
Lastly, he gains the Mind Gem from the Grandmaster whom he beats at his own game in which all 6 gems were wagered
(End of Thanos Quest)
In a post-story, it shows Drax finding Thanos on a random planet but unable to have his vengeance after Thanos gets away by using innocents as collateral who Drax saves