WARNING: The following article contains spoilers for Avengers: Endgame, in theaters now.
Avengers: Endgame uses time travel as the driving, and somewhat controversial, factor of its plot, with Earth's Mightiest Heroes returning to key moments and locations in the Marvel Cinematic Universe to collect the Infinity Stones from the past -- all while not disrupting the time stream.
But as the Avengers split up for their "time heist," we're also given insight into earlier films, which has the side effect of filling in a couple of important plot holes.
THE ANCIENT ONE'S AFFINITY FOR ALL THINGS STRANGE
After Mark Ruffalo's Hulk travels to the Battle of New York, from 2012's The Avengers, he heads to the New York Sanctum to retrieve the Time Stone. There, he encounters the Ancient One (Tilda Swinton), as he's four years too early to meet Stephen Strange. However, she refuses to lend him the Time Stone, until he mentions Doctor Strange gave the cosmic relic to Thanos during the events of Infinity War.
She buys into this endgame her successor will foresee, and gives Bruce the Infinity Stone. However, she makes it clear her decision is based on her trust of Strange, not Hulk. The Ancient One realizes how important Strange is to the future of all reality, and indicates he's destined to be "the greatest Sorcerer Supreme of them all."
The exchange informs her action in 2016's Doctor Strange, in which it seemed odd that she would select a random, self-absorbed stranger as her successor. But clearly, before her death at the hands of Kaecilius, The Ancient One already knew Strange was destined to be Sorcerer Supreme. Her conversation with Bruce closes the loop, and confirms how she knew of Strange's potential before he even stepped foot in Kamar Taj. It wasn't clairvoyance or some sort of prophecy; Endgame reveals a large part of her belief in Stephen Strange was because of what Hulk hinted about his actions in the future.
HOW HYDRA GOT THE MIND STONE
In the post-credits scene of 2014's Captain America: The Winter Soldier, we saw Baron von Strucker and his Hydra lab in Sokovia, using Loki's scepter and its Mind Stone for experiments on humans. It was the origins of Elizabeth Olsen's Scarlet Witch and Aaron Taylor-Johnson's Quicksilver. However, before that the scepter was last seen at the end 0f 2012's The Avengers, placed by Black Widow in S.H.I.E.L.D. custody. However, w now know what happened to the weapon as the Avengers muzzled Loki for talking too much and began making plans for shawarma.
In Endgame, when the future Tony Stark sneaks into Stark Towers and spies on the shackling of Loki following his beatdown by the Hulk, he sees Black Widow place the scepter in the care of Agent Jasper Sitwell and a team led by Brock "Crossbones" Rumlow. Tony is tasked with acquiring the Tesseract, so he alerts Captain America to intercept the scepter in the elevator. In an homage to The Winter Soldier, Steve pretends to be a Hydra operative like them and takes the case, indicating he'll deliver it to Robert Redford's Alexander Pierce.
So now we know that in 2012, without interference from the future Steve Rogers, Sitwell's Hydra sleeper agents abscond with the scepter, intent on taking it to Sokovia. The sequence retcons the Avengers: Age of Ultron Prelude - This Scepter'd Isle comic, which indicated the Mind Stone went into S.H.I.E.L.D. care. However, Endgame suggests Sitwell had Rumlow's team deliver the scepter to von Strucker.
Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Mark Ruffalo as Bruce Banner, Chris Hemsworth as Thor, Scarlett Johansson as Black Widow, Jeremy Renner as Hawkeye, Brie Larson as Captain Marvel, Paul Rudd as Ant-Man, Don Cheadle as War Machine, Karen Gillan as Nebula, Danai Gurira as Okoye and Bradley Cooper as Rocket, with Gwyneth Paltrow Pepper Potts, Jon Favreau as Happy Hogan, Benedict Wong as Wong, Tessa Thompson as Valkyrie and Josh Brolin as Thanos.