The following post contains massive spoilers for Avengers: Endgame

The biggest scene in the history of the Marvel Cinematic Universe occurs at the conclusion of Avengers: Endgame, bringing together 21 movies and the most powerful superheroes in the galaxy to celebrate the life and ultimate sacrifice of Tony Stark (Robert Downey Jr.). As the camera moves through attendees at Stark's funeral, audiences see Ant-Man, the Wasp, Doctor Strange, the Guardians of the Galaxy, Thor, Hawkeye, Captain America, Scarlet Witch, Hulk, Black Panther, Captain Marvel, Nick Fury, and... Harley Keener?

Wait, who is Harley Keener?

Everything to Know About the Avengers: Endgame Credits

Played by Ty Simpkins, Keener is a key character in Iron Man 3, a young boy who helps Tony rebuild his Mark suit after Iron Man is feared dead. In that film, co-written and directed by Shane Black, Keener matches Tony quip for quip and receives some good advice from the billionaire inventor on how to handle a school bully.

Simpkins was 11 when Iron Man 3 came out in 2013 and said at the time that he loved working with Downey in the film. "[It] was cool. He stayed in characters the whole time, but he was really nice, and he made it easy to work with him," Simpkins told Coming Soon.

While his appearance in Avengers: Endgame is a surprise — especially considering the heavy hitters who also feature in the sequence — Simpkins' casting was hiding in plain sight all this time. His involvement in Avengers: Endgame was revealed all the way back in 2017. (Simpkins also walked the red carpet before the film's premiere in April.)

Ty Simpkins attends the world premiere of Marvel's <em>Avengers: Endgame</em> on April 22, 2019 in Los Angeles.Ty Simpkins attends the world premiere of Marvel's Avengers: Endgame on April 22, 2019 in Los Angeles.

Speaking to Entertainment Weekly, Endgame directors Anthony and Joe Russo explained why Harley appeared at the memorial service. "Clearly Tony had an effect on him," Joe Russo said.

As for the scene itself, which happens after Tony uses the Infinity Stones to snap Thanos and his horde of evildoers out of existence for good — a move so powerful that it kills Tony in the process — actor Sebastian Stan (who plays Bucky Barnes and is present during the funeral) teased back in 2018 that it took months for Marvel to put together.

"There was one scene, I think, we had where everybody was there. I can't really talk about that scene, but I knew it took them three months in planning this scene to have everyone there," Stan said at an event. "You look around and you just saw everyone from Samuel L. Jackson to Michael Douglas to Michelle Pfeiffer. Everybody was there."

Including Ty Simpkins.

Avengers: Endgame is out in theaters now. Iron Man 3 is streaming via Amazon, Vudu, YouTube, Google Play, and iTunes.