It details the escapades of two high school teachers, 35-year-old Saeko Yamaguchi (Miki Yamaguchi in the English version) and 24-year-old Mayumi Hachiya, as they proceed to give their student body the full sex education course.
The story of Secret Plot (and its sequel Secret Plot Deep) revolves around Miki and Mayumi's sexual escapades and those that get caught up in it. Early on Mayumi by complete accident finds out that one of her senior high school students, Masaki Kobayashi, has a twelve-inch (305 mm) -long, three-inch (76 mm) -thick penis. She uses her power as a teacher to "educate" him, but their relationship continues after he graduates.
Miki is involved with two virgin seniors early on, but they graduate and she is left wanting. Eventually Mayumi tells her about her student with the "gift", Masaki, and she and Miki make a deal that Miki can sleep with him, but only if she pleasures Mayumi. Mayumi sets up Masaki to be found by Miki while masturbating, and Miki then proceeds to have bondage sex with him.
They find Masaki at his apartment later on, and they find out that Masaki has been masturbating even though Miki and Mayumi suck him dry every day. They force him to have a threesome with them and then make him promise never to masturbate.
Eventually the three get together in a threesome in the last volume, with the plot left open for a sequel.
Secret Plot: We are introduced to 24-year-old Mayumi Hachiya, the P.E. teacher at the same high school as Yamaguchi-sensei. She calls aside one of her students, Masaki Kobayashi, and demands to know why he skipped class for an entire year; he said it was because of his over-sized penis (for which he was picked on in elementary school) so he couldn't wear gym shorts. Verifying this, Mayumi proceeds to make good use of it - during and after graduation.
Hard Exercise: This episode centers around Masaki and his "private lessons" at Mayumi's apartment. Mayumi sets up a video camera to record the P.E. "class", then uses the tape to blackmail him into having class every day.
Fist Crusher: Mayumi informs sexually-frustrated Miki of her twelve-inch (305 mm) -long, three-inch (76 mm) -thick toy-boy and is willing to share - in exchange for some immediate lesbian action. While Miki fists Mayumi, the athletic P.E. teacher's vaginal muscles crush Miki's wrist.
Lonely Woman: Beginning with a harem-style fellatio-orientated dream, Mayumi wakes up, wards off a door-to-door anti-porn spokeswoman, then masturbates in the shower while washing the door-knocker's blood off - before slipping and knocking herself unconscious on the bathroom floor.
Twin Bitches: Masaki runs late from university to his private P.E. "lessons" with Mayumi (she needs him to change into his high school uniform), this time at the high school. Mayumi arouses him in an empty laboratory, then leaves him behind with instruction to wait until she returns; a ruse to allow Miki to have her turn. Miki "catches" him in the act - a university student masturbating at his old high school, in his old uniform (albeit with his pants off) - and threatens to call the police. Masaki pleads that he'll do anything to avoid prison (not knowing he was set up) so Miki has him go down on her, then threatens even worse punishment.
Riding Punishers: The continuation of Twin Bitches, Miki has sex with Masaki while Mayumi watches the whole thing in the control room via surveillance camera. At the end, Mayumi shows Miki the tape she made of the event but, instead of just giving her the tape, Mayumi offers to not show it to the principal in exchange for dinner at the "good, really expensive sushi place down the road" because she is "very, very hungry" (Miki sobs for her poor credit card).
Powerful Visitors: Masaki's university roommate tells him that he'll be spending the night with a new girl, and to tell his girlfriend (Hiromi) that he is "staying with his folks". Interrupted from his (microwaved) dinner by the doorbell, he find Mayumi and Miki, completely drunk, have arrived for a "teacher/student conference". They proceed to consume all the food and booze in the place - including his dinner and his roommate's single malt scotch - and discover his hidden porn collection; Mayumi even discovers a waste-paper basket full of "used" tissues, even after all she does for (or, rather, to) him. Miki poses herself like one of the models in Masaki's magazines but Mayumi says she's too "loose and baggy", proving it by ramming her fist (and half of her forearm) straight up her vagina and fists her to orgasm. Noticing Masaki's erection, Mayumi insists that he is forbidden to masturbate and if he needs to "relieve" himself he is to come to her at the high school. The chapter ends with Mayumi and Miki fighting over who gets to have sex with him first.
Milk Drinkers: The conclusion of Powerful Visitors (and the Secret Plot series) begins with the who-goes-first argument between Miki and Mayumi heating up. When Masaki asks them to stop fighting, they do - deciding to have a threesome instead; leading Masaki to make an unsuccessful (and futile) attempt to escape. After three bouts of hot, sweaty, boy-on-girl-on-girl sex, an exhausted Masaki discovers that the two teachers had just finished their warm-up and were now ready to get serious. Waking up hours later, he finds a thank-you note from his teachers attached to a "proper" meal to get his strength back... for the next time.
T.V. Report: At the end of the school day, Mayumi is hit by a sudden downpour on her way home and takes refuge in a public toilet. She is surprised when a girl from her school leaves the only cubicle - even more so when she recognizes "her" as Yuri Imada, one of her male students. Preventing him from leaving, she discovers Yuri had an older sister, Yuko (who looked almost identical to him) who died in a car crash the previous year, driving his parents to alcoholism and unemployment. In a despair- and alcohol-induced breakdown his parents mistook Yuri for Yuko, leading them to believe it was he who had died - Yuri has had to become "Yuko" for them ever since, changing from one uniform to the other in the public toilet before and after school. Yuri distraught at having been found out, Mayumi decided to "solve" his problem by inserting a double-ended dildo into herself and proceeding to turn him into a "girl" through sodomy.
Nightmare: "Yuko" finds himself being attacked by a gang of rapists and tells them he is a man, a point unmade by the presence of a female body when they strip "her" naked and rape him. Beginning to enjoy the experience, he is shocked awake from his dream to find he came in his sleep.
True Vision: Brought out of a nice bath by the doorbell, Mayumi is ready to clobber the "God-pushing skank" again (from the Lonely Woman arc of Secret Plot) and finds "Yuko" (nicknamed "Yuya") scared out of his mind by her threat. Letting him in, Mayumi guesses (correctly) that Yuri wanted to be treated like a girl again. This time, Mayumi makes him beg for it and insists he get on top. A week later, "Yuko" shows up again at Mayumi's apartment asking for another ass-fucking, and that he wants to ass-fuck her, too.
Behind the Glasses: Yuri runs late in the rain for a date with Mayumi; reluctantly wearing a short skirt with no panties as she had demanded. Yuri accidentally collides with a girl riding a bycilcle who turns out to be his classmate Mari. After she recognizes him by noticing his penis, Mari takes him to her apartment to dry off. She asks him about his attire,whereapon Yuri explains about the car crash and about his increasingly frustrating relationship with Mayumi. He gets emotional, asserting that had he been the one killed in the car crash, nobody would have cared. Mari begs to differ. She has been secretly in love with him, but was too shy to say anything. He apparently has feelings for her as well. They proceed to have sex for over an hour with Mari having multiple orgasms, while Yuri unable to climax. Apparently, due to his sessions with Mayumi, he can no longer achieve orgasm without anal stimulation. Mari expresses a desire to provide it, and soon Yuri ejaculates in her mouth. The rain eventually lets up and the sun shines upon the couple. They bask in post-orgasmic bliss holding hands; Yuri completely forgets about Mayumi, leving her alone and waiting at the agreed-upon location.
Little Heart, Big Body: Running down the hall with an armful of soda can, a chubby bespectacled male student (Kazu) trips and is saved by Miki's "airbags". They meet again after school when Miki saves him from the rain on his way home in her car, and she learns that his umbrella was "borrowed" by another student and the soda he bought was for someone else, too. Miki decides to "nobly sacrifice [her] body to give a young man self-esteem and the courage to fight back" (meaning, "use sex as a tool to enslave a cute young man and make him [her] personal fuck-toy") - almost crashing due to her daydream, her "airbags" save him again. At her apartment she insists Kazu takes a hot bath in order to not catch cold, then joins him to wash his back. Seducing him into self confidence, Miki teaches him how to make love (she not as sexually aggressive as Mayumi) and asks him to remember her the next time he needs to stand up for himself. Unfortunately, the next day Kazu is back suppling soda, Miki discovering it was Mayumi who was bullying it out of him.
Atomic Beach: Mayumi invites Miki and Masaki to an unknown deserted beach, riding in Miki's 4WD (the G.P.S. was broken because Mayumi beer-puked on it). Wondering why such a perfect beach was unknown to everyone else during the summer holidays, Masaki was answered by a giant sea slug dropped at his feet by Mayumi, who learned of the beach being "clean" and "safe" from a newspaper report about a nearby nuclear power plant! Miki wanting to swim more (Mayumi saying a little more exercise won't firm up her sagging breasts but her implants might soak up some brine), Masaki was made to massage sun oil into Mayumi's body, ultimately leading to an oily threesome on the sand. At the end, Miki complained about being the designated driver but was comforted by Mayumi and Masaki falling asleep on each other.
Secret Heart: Yuri and Masaki are talking about a character-fighting computer game (about buxom, scantily-clad female furries) after Masaki finished tutoring Yuri for the calculus test when Mari (from Secret Plot Deep #4), now Yuri's girlfriend, picks him up for their shopping trip. A little depressed by the pair into wondering about the true nature of his relationship with Mayumi, she depresses him even more by saying that she does love him but it's his super-penis she loves the most. Finally able to get it up (and in) due to anger, Masaki fucks Mayumi hard enough to knock her head off the headboard. Regaining consciousness, Mayumi frightens Masaki by not knowing who he is.
Secret Hearts: The continuation of Secret Heart, Masaki learns from the doctor (at the St. Viagra Hospital!) that Mayumi has amnesia from the blow to her head and that he should allow her memories to return on their own, if they do at all. Feeling it was his fault, he tells her the doctor said her memories would come back in a few days. He drops her off at her apartment but Mayumi asks him to stay "a little longer". They look through the photos taken during their beach trip the previous month (at Atomic Beach) and she asks if Miki is his sister; at the one of them asleep in the back of Miki's car, Mayumi lays her head on Masaki's shoulder, wraps her arm around his, and subsequently falls asleep. She wakes up in the middle of the night to find to apartment in darkness and Masaki gone; when he comes back (having gone to the corner store for food) she hugs him and asks him to never leave her alone. While she is frightened by the loss of her memories, Mayumi says the only thing she knows for certain is that she loves him. As a response Masaki gently kisses her, and they proceed to make love (unlike the previous hard-core fucking). At the climax Mayumi loses her balance and her face hits the floor. Masaki find her with a bloody nose and her memories returned; Mayumi, not remembering thing from the amnesiac period, takes his joy as a sign that "[he] can fuck [her] forever and ever!!"