Steve and Natasha landed on Vormir with their mission in mind. Get the Soul Stone. Simple enough. Nothing could prepare them for what was coming next.
"So where is the Soul Stone?" Natasha asked.
"Welcome Natasha, daughter of Ivan. Steve, son of Sarah" Red Skull said.
Natasha was startled.
"Red Skull?" Steve said, confused. "You're alive? How?"
"You know him?" Natasha asked.
"Yeah" Steve said. "Nat, meet Johann Schmidt, better known as Red Skull. Not a great dude."
"Good to know" Natasha said.
"I am the Stonekeeper" Red Skull said.
"The what?" Steve asked.
"The Stonekeeper" Red Skull said. "I am keeper of the Soul Stone. I guide others to the treasure I cannot possess."
"Great" Natasha said. "Where is it?"
"Are you sure you wish to possess it?" Red Skull asked.
"Yes" Steve said. "Can we have it?"
"I do not have it" Red Skull said.
"Enough with the runaround" Steve said. "Where is the stone?"
"Over there" Red Skull said, pointing to the mountain surrounded by water.
Natasha and Steve looked toward the direction Red Skull was pointing to see a faint orange glow.
"That's it" Natasha said.
"I must warn you" Red Skull said. "In order to take the stone, you must lose that which you love."
Steve and Natasha looked at each other.
"An everlasting exchange" Red Skull said. "A soul, for a soul."
"What are you saying?" Natasha asked.
"The Soul Stone requires a sacrifice" Red Skull said.
Natasha sat down on a rock near the edge of the cliff.
"There has to be another way" Steve said.
Red Skull floated away.
Steve walked over to Natasha.
"One of us has to die" Natasha said.
"We'll figure out another way" Steve said. "We are both leaving here with the stone."
"You heard Red Skull" Natasha said. "It doesn't work that way."
"I don't trust Red Skull as far as I can throw him" Steve said. "How do we know he is even telling the truth?"
"Natasha, daughter of Ivan" Natasha said.
"Yeah" Steve said, confused.
"He knew my father's name" Natasha said. "I didn't even know that."
"So we trust that he was telling us the truth because he knew the name of your father?" Steve said. "No offence, but do you know how crazy that sounds?"
"Maybe" Natasha said. "But, what I do know is that Thanos left here with the stone and without his daughter. That can't be a coincidence."
"There has to be another way" Steve said, tears in his eyes.
"There isn't" Natasha said. "Whatever it takes, remember?"
Steve nodded. Natasha stared down the cliff.
"It's been nice working with you" Natasha said.
"Hold on" Steve said, pulling her away from the edge of the cliff. "What makes you think you're the one sacrificing yourself?"
"I have to clear the red in my ledger" Natasha said. "Plus, the world needs Captain America. The world does not need Black Widow."
"You really believe that?" Steve asked.
Natasha looked at Steve with tears in her eyes.
"The world needs you. The Avengers need you" Steve said. "You never gave up. That fight in you. That fire. That's what the world is going to need going forward. I've always been out of my time. The one woman I have loved is gone. She had a life without me, but I would like to be reunited with her."
"Steve" Natasha said, tears streaming down her cheeks.
"It's going to be okay" Steve said. "You are going to continue to do great things. You are going to continue to be the hero the world needs. Natasha Romanoff, Black Widow. Take the stone back and bring our family and friends back."
"I can't say goodbye" Natasha cried.
"I'll be okay and so will you" Steve said. "This is what has to be done."
Steve cupped his hands over Natasha's cheeks and kissed her forehead.
"I love you" Steve said.
"I love you too" Natasha said. "I hope you are reunited with Peggy."
Natasha followed Steve to the edge of the cliff. Steve looked at Natasha, both of them fighting back tears. With a slight nod, Steve jumped to his death. Natasha fell to her knees and cried. She opened her hand and saw the Soul Stone glowing in her hand.
"We'll save them all, Steve" Natasha said. "I promise."
Natasha clutched the stone in her hand as she headed back to the ship.
A/N: So I thought about how Natasha would have been the one to leave Vormir alive and I think the only way she would have let someone else sacrifice themselves is if it was Steve.