The person at the door is Vanessa

Cynthia opens the door and is shocked to see Vanessa standing there, holding a shopping bag which Cynthia doesn't recognize.
"Oh! Van! What are you doing here?" Cynthia asks.
"You know that guy I told you about?" Vanessa asks, walking past Cynthia into the house, "The one who couldn't get it up?"
"Sure," Cynthia says, cautiously. Last time Vanessa confided in Cynthia about her sex life, it had led to them having sex - or not having sex, according to Vanessa and Mark. Where was this going?
"Well," Vanessa says, giving Cynthia her best puppydog eyes, "I need you to do me another favor."
"Is it going to be the same as the last favor I did you?" Cynthia asks.
"Not exactly!" Vanessa assures her hurriedly, "Look, you know I can't get off from masturbation. My boyfriend, he really tries, but he can't stay hard long enough to really fuck me. We have such a good thing going, and I don't wanna break up with him or fuck some random guy behind his back, but I need that feeling of being fucked, you know, of being really pounded into."
"I can't help you there, Vanessa, I don't have a dick," Cynthia points out. Vanessa smiles and holds out the shopping bag. Taking it from her and looking inside, Cynthia realizes what's inside: a purple dildo, lube, and a harness. Cynthia looks up at her friend.
"What the fuck, Vanessa? What do you want me to do with this?" Cynthia demands. Vanessa rolls her eyes.
"Oh, come on, Cynthia. What do you think I want you to do?" Vanessa says, as though Cynthia's an idiot for asking. Cynthia takes out the dildo, staring at it. It's mostly curved and mostly smooth in a way that makes it clear that it's not too interested in mimicking the shape of a penis. Cynthia blushes. She's never seen a dildo in person before, much less held one, much less been asked to wear one.
"Van, if you're gay or something, you know you can tell me, right?" Cynthia asks. Vanessa groans, rolling her eyes again.
"Jesus, Cynthia, it's not like I'm saying I want us to buy a Subaru and scissor in it," she says exasperatedly, "I'll be pretending you're a guy the whole time." Cynthia doesn't know that that makes it much straighter, but whatever. Vanessa takes Cynthia's hand and looks at her with those same pleading eyes.
"Come onnnn. If you don't do this for me, it's seriously gonna ruin my relationship," Vanessa begs, "I really like this guy." Cynthia sighs.
"Okay, I'll do it," she says. Vanessa grins, clasping her hands together with delight.
"I knew you would! Oh, you're such a good friend," Vanessa says, hugging her, "Okay, you get ready, and I'll be waiting for you in bed." Before Cynthia can stop her, Vanessa heads off to the bedroom. Shit. I sure hope Roussard picked a good place to hide, Cynthia thinks. She follows her friend into the bedroom. Fortunately, she doesn't see any sign of Roussard as she heads into the ensuite.

She takes the harness out of the bag.
"Uhh, do you want me to get naked or...?" she asks Vanessa through the door.
"Just do whatever feels natural," Vanessa replies. Natural. What part of this is meant to feel natural? she wonders. She tries putting the harness on over her sweatpants, but it makes the sweatpants bunch up in a way that looks weird and awkward. Instead, she takes off her sweatpants but leaves her panties on under the harness. She also chooses to leave on the t-shirt she's wearing. She gazes in the mirror, gawking at the sight of herself with a purple erection.
"Okay, I'm coming out," she tells Vanessa, "don't laugh." She opens the door.

Vanessa is on the bed. She's naked, and her body language welcomes Cynthia towards her. Cynthia takes a deep breath, her heart racing as she joins her on the bed. Nervously, she reaches out and touches Vanessa's thigh, feeling her soft skin beneath her palm as she moves her hand along it. She isn't sure how to proceed. With her other hand, she cups one of Vanessa's breasts, circling the nipple with her thumb. Vanessa reclines and Cynthia follows her, so that she's basically lying on top of her friend. Realizing that Vanessa's other nipple is now close to her face, Cynthia is seized by the unexpected desire to take it into her mouth. She does so, and Vanessa gasps with pleasure as Cynthia flicks her hard nipple with her tongue. Vanessa moans, and without relenting with her tongue, Cynthia lets go of the other breast and reaches down to Vanessa's pussy. She dips her fingers inside, coating them with wetness before caressing Vanessa's desperately hard clit. Vanessa cries out in pleasure as Cynthia directs her attention towards her clit, rubbing it up and down, loving the feeling of it between her fingers.

First the call with Roussard, then the teasing while she and Roussard had set up the surveillance, and now this - all day, desire has been mounting within Cynthia, growing harder to resist. Even though she's only meant to be doing a favor to Vanessa, she can't deny that this feels good. As Vanessa moans, writhing passionately against Cynthia's hand, Cynthia humps the bed with the dildo, doing all she can not to reach down and touch her own clit. She loses herself in her lust as she feverishly rubs Vanessa's clit, her mouth still planted around her nipple. She begins to feel like she might cum from this, even without touching herself. This makes her think about Roussard, how she'd made him cum untouched. She wonders where he's hiding, and whether he can hear Vanessa's moans as they grow higher and louder. She's so caught up in her own arousal that she barely notices Vanessa's legs shaking, her body growing tenser until it finally reaches a peak. Vanessa cries out, and her wet cunt spasms against Cynthia's fingers as her orgasm hits her.

"Damn," says Cynthia, slowing down her strokes of Vanessa's clit, "I only meant to finger you for a bit before I started with the dildo. I guess I got carried away." She gazes up at Vanessa, who grins at her, panting.
"Remember Joe Baumann, that boy I dated for a bit in college?" she asks. Cynthia nods, and Vanessa continues, "Well, he was a virgin, and we decided that we wanted to have sex, right? But when we got close, he was always too shy to take charge and fuck me, so we'd just make out. Finally, we got to the point where we were naked in bed together, and I put his hand between my legs and told him I wanted him to fuck me, but he was like 'just a minute, not so fast', but then after awkwardly fingering me for a minute while we made out, he came on my leg. That's what you reminded me of just now, getting so caught up in fingering me that you forgot to fuck me," Vanessa teases her, stroking Cynthia's head, "You're just like a nervous virgin boy."

Cynthia frowns. The way Vanessa talks about it, you'd think that Cynthia had been the one to get off just now, not Vanessa. Cynthia begins to sit up, moving away.
"Where do you think you're going?" Vanessa asks, pulling her back towards her, "Just because I came once doesn't mean I've gotten what I need. I got you the strap-on because I need to be fucked, not just fingered." She spreads her legs. Cynthia swallows anxiously, feeling just the way poor Joe Baumann must have when he was so nervous he ended up blowing his chance to sleep with Vanessa all those years ago. She takes the lube, squirts some along the length of the dildo and spreads it out with her hand, even though she knows that Vanessa is so wet they probably don't need lube at all. Vanessa strokes her pussy as she presents it to Cynthia, who feels almost light-headed with lust.

Finally, Cynthia grabs the head of the dildo and lines it up with Vanessa's opening before pushing forward. Vanessa moans, lifting her hips to take more of the dildo's length as Cynthia sinks into her. For a moment Cynthia is motionless.
"Fuck me," breathes Vanessa. Cynthia begins to tentatively rock her hips. Vanessa gasps and moans with each forward movement, obviously loving the feeling of the dildo penetrating her wet pussy. As Cynthia begins to gain confidence, her thrusts grow harder, faster. Vanessa's tits bounce with each thrust, and Cynthia can't help but be further aroused by their movement. She grips Vanessa's hips, drawing her close against her as she rams deeper into her eager cunt.
"Yes, ooooh, yes," Vanessa says, her voice high and strained, "oooooh Mark, fuck me."

Vanessa doesn't seem to notice Mark's name slip through her lips, but Cynthia does. For a moment, she'd forgotten that this woman was her enemy, that she'd betrayed Cynthia and didn't even seem to care. She'd forgotten that the reason she was wearing this strap-on was because she'd made Mark's cock too soft and unreliable to please Vanessa. A mixture of hatred towards the both of them and pride in her **** plot courses through her.
"Tell me again," Cynthia pants, "tell me again how he can't please you."
"He's useless," gasps Vanessa, her voice a lust-filled whine, "His cock doesn't stay - ooooOOooh, fuck, yes - hard long enough to fuck me."
"Beg me to make you cum," instructs Cynthia, "Beg me to make you cum the way his pathetic cock can't."
"Please, please, his cock is so pathetic," Vanessa begs, "Please, I need you to - oooaaaooh - make cum on your cock. Fuck, he's so pathetic, please, he can't- aaaaaAAAH!" Vanessa tenses up, her hips bucking as another orgasm hits suddenly, interrupting her. As Cynthia pushes forward, burying the dildo as deep as possible in Vanessa's pussy, the unexpected happens. A sudden wave of pleasure rises up from between her legs, exploding through her body. She moans, pressing her body flush against Vanessa's, her hips jerking of their own accord as she cums.

As the two women come down from their orgasms, Vanessa looks at Cynthia curiously.
"I can't believe you just came," Vanessa says, "You weren't faking or anything?"
"Nope, all real," Cynthia confesses, "I guess I've been a bit pent-up, too. Lately, Mark's been... well, he's always horny, and he gets hard fine, but he has no stamina. I get maybe a few pumps before he's cumming, and he never bothers to finish me off. I guess it was nice to be in control for once."
Vanessa gets up and heads to the bathroom. Cynthia takes off the harness. Her panties are soaked. When Vanessa comes back out and gets dressed to leave, Cynthia awkwardly tries to hand the dildo and harness to her.
"Oh! No, that's yours to keep," Vanessa says, "I can't cum from masturbating, so the dildo's no use to me without a person controlling it. Do whatever you want with it. That being said, don't throw it out. Today was nice. I might come back for round two sometime."
Cynthia protests, saying she has no use for it either, but it's no use. Soon, Vanessa gives her a kiss on the cheek and leaves.

Cynthia returns to the bedroom and sits on the bed. She can hardly believe what just happened. She can hardly believe she liked it so much. Before she has time to contemplate what this might mean for her, she is startled by a voice.
"Is it safe to come out now?" it whispers. Roussard!
"It's safe! Where are you?" she asks to the room at large, jumping off the bed. She sees a foot poke out from under the bed, followed by a leg, as Roussard shuffles out from his hiding place. Cynthia blushes at the realization that Roussard was less than two feet away when she and Vanessa were fucking. Roussard's hands cover his crotch, and when his face appears, it wears a sheepish expression.
"I'm really sorry," Roussard says, sounding dazed, "I wouldn't have hidden under the bed if I'd thought..."
"It's fine," Cynthia assures him, "I didn't see it coming either." Roussard sits up, but seems to realize that he can't stand up from this position without removing his hands from his crotch. As he moves his hands away, Cynthia sees that the panties she gave him to wear are soaked with cum. He notices her looking.
"I wasn't jerking off, I promise," he says, shame-faced, "but, well, between the panty kink, and the lowered stamina, and the sounds... It was all I could do to stay as quiet." He wobbles a little as he stands, his knees weak. Cynthia holds back a grin at the thought of him, trapped in the darkness under the bed, struggling not to make a sound as his shrunken cock ejaculates into the panties that belong to her.
"How long did it take, do you think?" she asks, "Before you came?"
"Honestly, it was embarrassingly fast," he confesses, "I came twice before Vanessa had her first orgasm. I don't know if the app did something weird to my refractory period or what, but I came four times, all without touching myself once. It was like nothing I've ever experienced."
"That's incredible," Cynthia says, "I wish I could've seen it."
"Maybe we can try doing something similar with the app later," he says, "In the meantime, I think I should probably get cleaned up before we finish setting up the tech. Do you mind if I take a shower?"
"Go ahead," she replies, "But wait a minute I'll bring you a towel."

As Roussard showers, Cynthia resets his cock. After hearing about what happened to him while he was under the bed, she's interested to see how many times in a row she can make Roussard cum handsfree with the app without even pressing the cum button, but that can wait for now. They've both had enough new experiences for one day.