Main couple - Chika (33): Long-haired woman. Impulsive, loudmouthed, and somewhat prideful. - Tatsuya (33): Chika's husband. Average income, average dick, and was bad at sex until Chika trained him.
Friend couple - Karin (28): Chika's friend. Shoulder length hair. Believes a woman's worth is determined by her partner's status. - Yukinari (26): Karin's husband. Rich, handsome, and incredible in bed.
Chika mocks Yukinari behind his back after Karin mentions she's never cum during sex. In reality Yukinari always makes Karin pass out from sheer pleasure before she can climax, but Chika doesn't let her explain and orders Tatsuya to finger Karin to show her what it's like to cum. Karin is secretly pissed that they disrespected her husband, so she tells Yukinari everything that happened and asks him to sleep with Chika to prove he's superior to Tatsuya.
They invite the main couple over and goad Tatsuya into having sex with Karin to "teach" Yukinari how to please her. This influences Chika into letting Yukinari finger her (and eventually more) later that night after Tatsuya falls asleep. When he wakes up, Karin leads him to a locked door where he hears Chika moaning after being trained by Yukinari's touch. Karin shows Tatsuya a live feed of the two as they progress into full-on sex. Karin gets horny watching Chika lose her mind to pleasure and lets Tatsuya hit again. Chika gets mindbroken by Yukinari's thunder cock and lets him cum inside, even though she's ovulating and trying for a baby with Tatsuya.
Basically Karin baited Chika into sleeping with her husband because she enjoys her super hax OP husband to fuck females around because it turns her on throwing her supposed friend under the bus. A fake excuse to get ntr from "swinging".