Jay (Toomtam) and Ploy (Mo) are boyfriend and girlfriend. Ploy is friends with a girl named Toey. Toey is dating a guy named Karn. A third girl, Pin, becomes involved in Toey and Karn's relationship. Toey gets cheated on. Ploy goes to talk to Pin, who is accused of being the third hand, and it turns out that she's actually too prim and proper to do such a thing, so the two become friends and grow closer. However, Ploy is still suspicious if she's a good person or not, so because she is so confident in her relationship with Jay and their love, she convinces him to try to court/date Pin to prove whether she's really innocent or not. Should love be something to test like this? Find out in "Ruk Long Jai".