Theft of the Space Stone

The Theft of the Space Stone was the attempt by the Avengers of acquiring the Tesseract from the past. After their mission in New York resulted in only the Mind Stone being retrieved, Iron Man and Captain America went back to 1970 to Camp Lehigh due to Stark knowing the Tesseract would be there as well as Howard Stark and other Pym Particles from Hank Pym, as they needed more in order to return to 2023.

First Attempt
Iron Man and Ant-Man tried to steal the Space Stone while Captain America stole the Mind Stone. Lang plants himself onto Stark's 2012 counterpart whom is transporting the Tesseract along with S.H.I.E.L.D. agents and the past Avengers. Meanwhile, Stark departs in the Iron Man Armor: Mark LXXXV. In the lobby area, Scott causes a distraction by interfering with 2012 Stark's Arc Reactor, allowing the present Stark to secure the Tesseract. However, as Stark attempts to exit the area, 2012 Hulk bursts into the lobby from the stairwell, and causing Stark to drop the Tesseract. In the confusion, Loki manages to retrieve the Tesseract and use its power to escape.

Second Attempt
Having failed to retrieve the Tesseract, Stark and Lang meet with Rogers, who successfully recovered the Mind Stone. With no other options and only enough Pym Particles for one more trip, Stark hatches an idea to recover the Tesseract as well as extra Pym Particles by traveling further back in time, deducing that in April 1970 both the Tesseract and Hank Pym were both present. Stark and Rogers put their trust in each other, and as Rogers hands Lang the Mind Stone and tells him to return back to 2023, Rogers departs with Stark to April 1970.

In Camp Lehigh in April 1970, an incognito Stark and Rogers immediately deduce where the Tesseract is being held and head to retrieve it. As they are both riding an elevator, Stark, posing as a doctor, leaves on an earlier floor to retrieve the Tesseract, and Rogers, posing as a military soldier, stays on the elevator on his own mission to retrieve more Pym Particles. Stark locates the Tesseract in a safe and silently breaks inside it, storing it in a suitcase, but before he is able to leave the scene he is confronted by his father, Howard Stark, who is unaware of who he is. Hiding his identity, Tony reveals his name is "Howard Potts" and he is a visitor from M.I.T., and the two greet each other.

Rogers calls Hank Pym in his laboratory and uses a ruse to lure him to leave, as he is able to successfully infiltrate and secure the Pym Particles. As Tony and Howard converse in an elevator, Howard points out that his wife is pregnant. Rogers prepares to leave but is forced to walk in an office to hide himself from military soldiers who are on alert from a potential breach. Rogers immediately notices an old picture of himself before being injected with the Super Soldier Serum, realizing he walked in Peggy Carter's office. Rogers then secretly spies on his old lover walk in another room through a window, who does not notice Rogers.

Tony and Howard continue to converse as they walk outside, as Tony notices Rogers gives him a thumbs up. Howard asks Tony for fatherly advice, and Tony answers while dropping subliminal comments about his own view of his father. As their conversation closes and Howard leaves to meet with his wife, Stark and Rogers succeeded in their missions and head back to 2023.

Aftermath
Stark and Rogers return back to 2023 with the Space Stone and extra Pym Particles. The Space Stone was later used with the rest of the stones to reverse the Decimation. After the Battle of Earth, Captain America goes back in time to return the Space Stone to right when it was taken.