Caution: Spoilers ahead for Avengers: Endgame.
The Ancient One makes a surprise appearance in Avengers: Endgame, but while her words of wisdom are usually worth heeding, the character was misinformed on this particular occasion. Avengers: Endgame picks up from the devastating cliffhanger of 2018's Avengers: Infinity War, which concluded with Thanos fulfilling his goal of wiping out half of all life in the universe. With Captain Marvel's timely arrival, the remaining Avengers team succeed in finding and executing Thanos, but that can't heal the overbearing fog of misery pulled over the world, nor bring back the billions that were killed by a snap of the Mad Titan's finger and thumb.
Thanks to a well-placed rat however, Paul Rudd's Ant-Man is able to return from the Quantum Realm, bringing fresh possibilities of time travel with him. With the aid of the science bros, Bruce Banner and Tony Stark, the Avengers hatch a plan to scatter throughout time, spirit away the Infinity Stones, return to their own time and finally, use the stolen stones to undo Thanos' snap. Realizing that, in 2012, three Infinity Stones were located in New York City (because why wouldn't three artifacts once scattered throughout the universe all end up in the Big Apple), the team of Captain America, Iron Man, Hulk, and Ant-Man head back into the climactic battle of the original Avengers movie to retrieve the stones.
Hulk is tasked with picking up the Time Stone from Sanctum Sanctorum, at that time in the possession of Tilda Swinton's Ancient One from Doctor Strange, who initially refuses to give the stone away. She tells Hulk that he can't have her Time Stone because it is required to act as a deterrent against inter-dimensional threats, and if the Avengers altered time, they'd create splits in the multiverse that would cause untold problems in other timelines and there'd be no Time Stone to help solve them. Hulk tries to negotiate by promising to return the stone to her at the same time as if it never left, but it's actually the mention of Doctor Strange that finally convinces the Ancient One to hand over the Time Stone to Hulk.
This scene is highly curious, as it opens up a huge can of worms regarding time travel and multiple dimensions in the MCU that are enough to give even the most ardent MCU fan a headache. It's also strange that a figure as wise and all-knowing as the Ancient One can't immediately see the bigger picture at play. The Ancient One has the ability to view past the barriers of time, so why can't she see that if the big green man in front of her doesn't get the small green stone around her neck, the universe will witness the greatest devastation it has ever known, something that far outweighs many of the threats the Ancient One is concerned by.
The answer to this can be found back in the character's 2016 Doctor Strange debut, in the Ancient One's death scene. Despite the best efforts of Strange and his medical ex-colleagues, the Ancient One is killed after a fight with the movie's main villain, Kaecilius, but before she departs the mortal plane, shares a heartfelt exchange with her successor, Dr. Stephen Strange. Here, the Ancient One reveals that, despite her omniscience, she is unable to see past a certain point - the moment of her death. Since this moment comes before the arrival and invasion of Thanos, she wouldn't have been able to predict the sheer destruction the villain would cause. The infamous snap is beyond her scope of reckoning.
The Ancient One may have known about Thanos' existence and perhaps even been aware that he was seeking the Infinity Stones, like many other beings no doubt, but she wouldn't have had any foresight of his ultimate victory or how rapidly he filled up the spaces in his gauntlet, and this partly accounts for why she trusts Strange's judgment over her own in Avengers: Endgame.
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