They've been dating for a few weeks when the issue of mutants comes up. William and Lizzie are having dinner with Fitz and Brandon and Gigi, and the topic turns to the increasing numbers of mutants in their population.
The first hint that something is wrong is that Lizzie is unusually taciturn about the subject. William knows that Lizzie usually is very vocal about her opinions, so after not hearing her chime into the conversation, he looks at her, and is shocked to see a sad expression on her face.
He leans over and murmurs into her ear, "Lizzie, what's the matter?"
"I...I'll tell you when we get home," she says quietly.
"Okay," he replies.
William enters the conversation again, and quickly manages to change the topic, which seemingly cheers Lizzie up a lot, and that always makes William happy.
When they get back to Lizzie's house, Lizzie tells William to wait in the living room. She quickly leaves and enters her dad's study.
She's in there for a few minutes, and when she comes out, she looks worried. "C'mon, Will," she says, and he follows her upstairs to her room. He sits on a chair as she walks over to her computer and turns it on.
Finally, she says, "Come here," and he gets up and walks over to the computer. On the screen is what appears to be a file of Lizzie.
"Read this part," she says, moving her cursor to a section titled "Ability(ies)".
It takes him a few seconds to absorb the words "Level 2 hydropath", and then it hits him with the force of a ton of bricks: Lizzie is a mutant.
Lizzie Bennet is a mutant. A telepath, to be precise, but still a mutant.
William can't think of anything to say, he's so shocked. No wonder she looked sad at dinner tonight!
"Will?" Lizzie's voice shakes him out of his reverie.
William reaches over and pulls Lizzie into a long, passionate kiss. When they break away for air, Lizzie asks, "What was that for?"
"That was because I love you," states William. "No matter that you're a mutant."
Lizzie smiles, and William returns it.