Warning: Major spoilers ahead for “Captain America: Brave New World.”

“Captain America: Brave New World” is filled with heroic moments from Anthony Mackie’s titular superhero, Harrison Ford transforming into the comic book character Red Hulk, and a surprise cameo from a fan-favorite Marvel character.

The fourth “Captain America” movie follows Mackie’s Sam Wilson after he inherited Steve Rogers’ (Chris Evans) shield and the title of Captain America in “Avengers: Endgame.”

In Sam’s first solo outing as Captain America, the everyman character struggles with the pressure and responsibility of saving the world despite not having superpowers or special abilities. But a visit from an old friend reassures him that he was chosen as Steve’s successor for a reason.

Here’s everything to know about the movie’s standout cameo.

Sebastian Stan returns as Bucky Barnes to give Sam Wilson a much-needed pep talk

Anthony Mackie as Sam Wilson and Sebastian Stan as Bucky Barnes in “The Falcon and the Winter Soldier.” Marvel Studios

Sebastian Stan was last seen as Bucky Barnes in the 2021 Disney+ miniseries “The Falcon and the Winter Soldier,” which costarred Mackie.

In “Brave New World,” Sam successfully stops rogue US fighter pilots from attacking a Japanese fleet. But in the process, his sidekick Joaquin Torres/The Falcon (Danny Ramirez) gets injured mid-flight and crashes into the Indian Ocean. After being rescued, Joaquin is immediately taken to the ER.

Sam, feeling guilty for what happened, watches through a window as doctors attend to Joaquin’s extreme injuries. Then, Bucky arrives in a suit and stands next to Sam.

“Hate to admit it, but I’m glad you’re here,” Sam says, as the two hug.

After Bucky tells Sam that Joaquin’s accident isn’t his fault, Sam replies, “It makes me think of Steve.”

The two talk about how Steve successfully stopped two alien invasions, and Sam, by comparison, feels unqualified and unworthy of being Cap.

“What made me think I could follow that?” Sam says.

Sam adds that he should have taken the super-soldier serum like Steve and Bucky did because all of this is bigger than him and his human limitations.

“Steve made a mistake,” Sam says.

“You’re a human being,” Bucky replies “You do your best. Steve gave them something to believe in. You give them something to aspire to.”

To emphasize his point further, Bucky concludes, “He gave you that shield not because you’re the strongest, but because you’re you.”

“Did your speechwriters help you with that?” Sam jokes.

Then, Bucky leaves for a political campaign obligation.

Once he exits, Ruth Bat-Seraph (Shira Haas), the security advisor to US President Thaddeus Ross (Ford), refers to Bucky as “future congressman James Buchanan Barnes.”

Stan’s cameo sets up his appearance in the next Marvel movie, ‘Thunderbolts*’

Sebastian Stan as Bucky Barnes in “Thunderbolts*.” Marvel Studios

Stan will return as Bucky in the film “Thunderbolts*,” set for release in theaters on May 2.

Eagle-eyed fans already picked up on Bucky seemingly wearing a US Congress pin in the most recent trailer for “Thunderbolts.*” His cameo in “Brave New World” tees up this surprising pivot from an assassin to a congressman.

“Thunderbolts*” centers on Bucky and a group of misfit Marvel characters who team up to stop a mysterious new threat.

The ragtag team of unlikely world-savers consists of Yelena Belova (Florence Pugh), Alexei Shostakov/Red Guardian (David Harbour), Ava Starr/Ghost (Hannah John-Kamen), John Walker/US Agent (Wyatt Russell), and Antonia Dreykov/Taskmaster (Olga Kurylenko). The movie also stars Julia Louis-Dreyfus as the enigmatic Valentina Allegra de Fontaine and MCU newcomer Lewis Pullman as Bob/Sentry.

“Captain America: Brave New World” is now playing in theaters.