It's a snowy Hogsmeade weekend in their seventh year. James is hanging out with the Marauders, Lily with her friends. Cue scary Death Eater attack. Lily gets separated from her friends, trips and ends up face-to-face (or more accurately face-to-knee) with a Death Eater. Wand gets lost somewhere in the snow. Lily freezes, the Death Eater freezes, time stands still until good ol' Prongs comes by to save the day. The Death Eater gets away and joins his friends who are wrecking havoc elsewhere.
Lily finds her wand and gets up. Before she can stutter an awkward thank you, James tells her that he has to go find his friends. Without even thinking, Lily says, "I'll help."
"Are you sure?" James asks, looking at her skeptically.
"Of course! Why, do you think I can't fight?"
"I never said that," James says defensively.
"Whatever, we don't have time to argue about this."
They don't have to look far. Sirius and Peter catch up with the pair outside of Honeydukes. Sirius tells them that Remus went back to Hogwarts to get help and that there's a group of third years having a Gobstones tournament in the Three Broomsticks. Lily internally debates splitting with the group to find her own friends, but decides the third years are in more danger.
They sneak in through a secret entrance that leads to the storeroom. It's decided that Peter, the stealthiest of the group, will gather the children and lead them to safety while the rest of them distract and overpower the Death Eaters. The plan goes off swimmingly. Injuries are dealt on both sides, but the trio prevails. By the time they get outside, the battle is done.
Six of the twelve Death Eaters are captured. They are children, some graduated, some not. None bear the Dark Mark. The official story in the Daily Prophet is that a group of teenagers tried to stir up trouble through a malicious prank. Few buy it, especially after Dumbledore awards points to those who fought and makes a speech about how their world will need people like them in the coming years.
After the battle, Severus Snape burns the Death Eater outfit and returns to the school. He tells Slughorn he was hiding out behind one of the stores. He wanders the halls aimlessly, haunted by terrified emerald eyes at the end of his wand. Severus stops when he hears familiar voices from around the corner. He peeks his head out and sees Potter and Lily sitting together, wrapped in shock blankets with mugs of hot cocoa neglected at their feet.
"I wanted to say thank you for what you did for me back there," says Lily quietly. Her voice is hoarse and her face peppered with little cuts, but Severus doesn't think he's ever seen a more beautiful creature. Truly an angel fallen from the skies and forced to walk amongst mere mortals. He swallows down the taste of bile when he thinks about how a few years ago it would have been him sitting with her.
"Don't worry about it. You would have done the same for me," says Potter.
"No, really," says Lily, grabbing Potter's arm. "If you hadn't - who knows what could have happened."
Nothing, Severus' mind screams. Nothing would have happened. I would never have hurt you.
"James, I don't know how I can ever repay you," she says. Potter smirks through a light blush.
"You could come with me next Hogsmeade weekend," says Potter. He gives her a cheesy wink and Severus knows that Potter is joking but he still wants to jump out and throttle the boy. All Severus can do is watch as Lily laughs and playfully hits Potter.
"I don't think we're going to be having very many Hogsmeade weekends," whispers Lily with a faraway look in her eyes, staring at the wall across from them. Potter swallows hard and tentatively wraps an arm around Lily. Severus fully expects her to push Potter away, but she leans in and lets him wrap another arm tightly around her waist. Potter buries his face in Lily's shoulder and she closes her eyes and places a hand on top of his. Severus can tell they're both crying and he just cannot watch any longer.
It's no more than a week after that the two of them get into a relationship. The school is abuzz with the news. Even in the Slytherin Common Room Severus can't escape the rumours and speculation. When Lily is asked what made her change her mind about Potter, she says with a laugh that James is her knight in shining armour. Severus hears some Hufflepuff girls giggling about it later and wishes he could forget who the dragon Potter slew was.