Disclaimer: I don't own Fault In Our Stars. In this story Gus is a healthy teenager and Hazel alone has cancer in her lungs.
Isaac and Gus looked at each other.
"Are you sure about this," Isaac asked.
"Absolutely," Gus said firmly, "Hazel's oxygen tank isn't working anymore. Without a transplant she will die. I refuse to even entertain such an idea."
"If you are donating one lung to Hazel," Isaac said without pause, "I'm going to donate the other one."
Dr. Maria looked at both boys.
"Are you sure about this," she asked, "Do you have any idea the risk you're taking? What about post op infection-"
"This is Hazel we're talking about. We are healthy. If we get an infection we can fight it," Gus said.
"You said Hazel was at a point where transplantation was her only option. We've had testing done already and with her not a viable candidate for a lung transplant we are giving her our lungs." Isaac added.
...
"Gus," Hazel said, "What... is... this?"
"It's a card. I got it for you. It says, "I love you with all my lungs."
"We both do," Isaac added.
"I love you guys too... with all of my heart."
She paused before adding, "My lungs are worthless."
"Not after tomorrow Hazel Grace," Gus said, "Tomorrow I'm donating my right lung to you."
"WHAT? You mean I'm going to have a functioning lung?"
"Two," Isaac said, "I'm donating my left lung to you."
Hazel was speechless and not just because it hurt to talk.
...
5:38 AM Hazel was put out. Gus and Isaac were put out 10 minutes earlier. Hazel's diseased right lung was taken out. Gus's healthy right lung was put in. Hazel's diseased left lung was taken out. Isaac's healthy left lung was put in.
Hazel was sewn up. She woke up at 12:18 PM. Gus woke up at 12:28 PM as did Isaac. It would take a lot of breathing exercises for Gus to learn to breathe with one lung. The same would go for Isaac.
Thirty years later Isaac died in a car accident. Gus and Hazel put on his grave A LUNGFUL OF LOVE. Hazel had been pregnant with her first child. They named her Eyzace in honor of their friend. When Gus and Hazel died years later they joined Isaac in heaven. On Gus's grave Hazel wrote, "You live on through me"