Casey (Spencer Bradley) and her husband, Shane (Lucas Frost), are waiting for Shane's stepmother, Marilyn (Jennifer White), to arrive. Shane's excited to see his stepmom, though Casey, on the other hand, is a little nervous about it. Casey admits that she doesn't feel like Marilyn likes her very much. Whenever they're in the same room, it always seems like Marilyn is competing with Casey over Shane's attention. Shane insists that's not true and that they just need to spend more time together, which is why he's glad Marilyn's visiting. Casey isn't sold but before she can say anything else, Marilyn arrives, lugging her suitcases through the door and greeting Shane with a kiss on the lips. Casey is taken aback by this but doesn't call them out on it. Though Marilyn is obviously smitten with Shane, it's clear that she harbors little to no goodwill towards Casey, whom she treats rather coldly.Over the next few days, Casey catches Shane and Marilyn in various seemingly lustful situations that make her pause. The first time, Casey catches Marilyn whispering conspiratorially in Shane's ear while sending her mocking glances. Shane insists they were just planning dinner, so Casey shrugs it off. The second time, Casey spots Marilyn checking out Shane's physique, though she doesn't confront them about it. But when she spots them a third time, caressing each other sensually on the couch, she can't hold her tongue any longer. 'What the FUCK are you doing?' she demands.Shane pulls away from his stepmom, an anguished look of shame carved out on his face. Marilyn, on the other hand, doesn't feel like she needs to hide anything, and proudly declares that she and Shane are sexually involved. Casey can't believe her ears- this is simply too much. How could Shane DO this to her? Shane explains that he tried to call it off after he got married to Casey, but whenever he's around Marilyn he just can't help himself. At the same time, he loves Casey and hates himself for not being able to control this taboo urge of his.Casey takes this all in and then realizes that no matter what she does, Shane will always crawl back to Marilyn. And if she can't compete with Marilyn, then maybe she can join her and Shane in their lovemaking. Marilyn is at first hesitant, but after Casey pleads with her to let her in, she allows it. They have an energetic threesome, with Casey doing all she can to save her marriage from the dominating grip of Marilyn.