"What If... Captain Carter Were the First Avenger?" is the first episode of the first season of the American animated television series What If...?, based on the Marvel Comics series of the same name. It explores what would happen if the events of the Marvel Cinematic Universe (MCU) film Captain America: The First Avenger (2011) occurred differently, with Peggy Carter taking the Super Soldier Serum instead of Steve Rogers and becoming the superhero "Captain Carter". The episode was written by head writer A. C. Bradley and directed by Bryan Andrews.

Jeffrey Wright narrates the series as the Watcher, with this episode also starring the voices of Hayley Atwell (Carter), Sebastian Stan, Dominic Cooper, Stanley Tucci, Toby Jones, Bradley Whitford, Ross Marquand, and Darrell Hammond. The series began development by September 2018, with Andrews and Bradley joining soon after, and many actors expected to reprise their roles from the MCU films. The episode shows Carter dealing with sexism while becoming a superhero. Animation was provided by Blue Spirit, with Stephan Franck serving as head of animation; they took inspiration from 1940s serial films and old war films.

"What If... Captain Carter Were the First Avenger?" was released on Disney+ on August 11, 2021. Critics praised Atwell's performance and the action sequences, but were mixed on other returning film actors' performances, elements of the animation style, and whether the story was a good introduction to the series or too similar to The First Avenger.

During World War II, Steve Rogers is chosen to become the world's first super-soldier by receiving Dr. Abraham Erskine's Super Soldier Serum. When Erskine asks if Strategic Scientific Reserve (SSR) Agent Peggy Carter wants to watch the procedure from a safe distance, she chooses to stay in the operating theater.

As Erskine and inventor Howard Stark prepare to put Rogers through the procedure, Heinz Kruger—a spy from the Nazis' science division Hydra—becomes nervous due to Carter's presence and attacks the lab before the transfusion can begin in the hopes of stealing the serum. He kills Erskine and SSR leader Chester Phillips and shoots Rogers before Carter kills Kruger. With limited time to complete the procedure, Carter volunteers to take Rogers' place. She is successfully enhanced, but the SSR's new leader John Flynn refuses to allow her to join the war because she is a woman.

Fifteen months later in Norway, Hydra's leader Johann Schmidt / Red Skull obtains the Tesseract, a powerful artifact that can manipulate space, with which he plans to win the war. Flynn refuses to send anyone to stop Schmidt, but Stark secretly gives Carter a costume and a shield made of Vibranium, which she uses to attack a Hydra convoy in Berlin and successfully retrieve the Tesseract and Hydra scientist Arnim Zola. Following this, Flynn reluctantly promotes Carter to a combat role and she becomes "Captain Carter".

Using the Tesseract, Stark builds a weaponized, armored "Hydra Stomper" suit for Rogers to pilot. Carter and Rogers go on to fight in numerous battles with the Howling Commandos until Rogers is presumed dead while attacking a Hydra train.

Upon obtaining its location from Zola, Carter leads Stark and the commandos in infiltrating Schmidt's castle, where they find Rogers alive. Intending for Hydra to continue in Nazi Germany's place following its collapse, Schmidt uses the Tesseract to open a portal and summon an interdimensional creature, but it kills him. Carter and Rogers battle the creature until the Hydra Stomper runs out of power. As Stark closes the portal, Carter sacrifices herself by pushing the creature back into it.