"The Cupid Effect" is the sixty-ninth episode of Kim Possible, and also the series' Valentine's Day episode.
Valentine's Day is imminent, and at Club Banana, Monique is complaining to Kim about a horrible drive-through date she had. She also reveals her jealously that Kim and Ron probably have something planned for Valentine's Day as she is stuck dateless, but Kim responds that Ron had not remembered about it and they had no plans. As if on cue, Ron arrives at Club Banana because Smarty Mart's pet department was closed for a while as some animal cages were left open and "food chain issues" occurred. Kim reminds him of Valentine's Day, but Ron tells her he never had a girlfriend on Valentine's Day, to which point he falls silent and realizes that this year he does have a girlfriend. Wade pops up in person and scares Ron by tapping him on the shoulder, grabbing the Kimmunicator because the batteries have been old and showing her the new X-ray plug-ins. Monique greets him for the first time in person and he suddenly falls in love with her. He wants to ask her out, but Monique does not like the age difference and he is too shy to try. Ron decides to help him out, going even as far as to say, "Who's dating Kim Possible?" to prove his credit as being able to help him.
After a failed attempt at the mall due mainly to Ron mixing up 'dating' with Abraham Lincoln and an unusual poem that he had written for class, Wade gets an idea from Ron to invent a "Cupid Ray" which would make Monique fall in love with him. Overnight, Wade develops the new device and builds a prototype. Just after he finished it, Ron entered with the idea they needed to take Monique somewhere romantic. Therefore, they bring Kim and Monique to France, Ron covering it by claiming that the Eiffel Tower is in danger. Fortunately for the two boys, Kim comes under the impression that Ron brought her to Paris for a date, which she finds romantic. As Kim pulled Ron away to talk to him about encouraging Wade, the young genius zaps Monique with the device casually and it works like a charm. Although confused, Kim believes that maybe Wade just needed a third chance and focuses on Ron for the night.
Unfortunately, Señor Senior Junior and his father are in France for "Le Goop" gel, and see the Cupid Ray in action from the store. Amazed at its capabilities and the potential to use it for evil, the two later steal the schematics from Wade's house while he is away and acquire advanced sound systems in order to amplify the Cupid Ray by roughly a hundred megawatts. Kim confronts Ron over the way Monique had suddenly fawned over Wade, and eventually forces Ron to confess about the Cupid Ray. Although disappointed in both of them –Wade for doing it, Ron for not trying to stop it– Kim focused on the fact that the technology was din the hands of the Seniors. Fortunately, Wade discovered that Junior was hosting a free for Ladies Only concert and the team prepares to leave when Monique arrived to confront Wade over what he had done. Needing to move quickly, Kim just brought Monique along for the mission.
Entering the concert itself proved to be difficult as Monique lacked a grappling hook like the others had. However, after dressing Wade and Ron as women, the four were able to enter the concert without issue. Aware that the Cupid Ray could effect Kim and Monique, Wade gave the two women special earplugs which would prevent the ray from having an effect on them. Junior arrives shortly after, and the crowd dislikes his singing until Senior activates the Cupid Ray on the crowd, who suddenly go from annoyed disinterest to raving over him like he was the best singer in the world. As the crowd went wild, Ron was accidentally knocked into Monique and hit her hearing protecting, leaving her again vulnerable to the Cupid Ray and she joins the crowd. Annoyed, Kim goes after Junior and confronts the confused junior villain as he did not understand why Kim was immune to the ray. Unfortunately for Kim, Senior was hiding behind her and removed her ear protection before she noticed he was there and fell under effect of the amplified Cupid Ray.
Desperate to stop their efforts from unraveling further, Ron goes after Junior personally while Wade leaves Monique in the crowd to go find the Cupid Ray. While Ron distracted everyone's attention by confronting Junior for luring Kim toward him, Wade snuck backstage and removed his feminine disguise. Wade was confronted by Senior, who introduced him to his two minions, Evil and Treachery. Fortunately, Wade evaded the two as they attacked him via his grappling belt and disabled the device, turning the Cupid Ray's effects on the concert attendees off and allowing them to run Junior out of the concert building as a mob.
Later at Bueno Nacho, Kim and Ron were having a simple date for Valentine's Day, but their kiss was interrupted by Monique reprimanding Wade over the whole incident. However, Monique was still happy with being a friend with Wade, she was simply not interested in a romantic attachment. Fortunately, Wade stumbles upon a love more within his stature in the form of Olivia, who was sitting a short distance away. The episode ends with Monique realizing she was dateless on Valentine's Day
Later, Wade explains to Ron that Olivia was a super genius herself and had invented a Cupid Ray of her own and had been using it on Wade. Ron momentarily freaks out, worried that Kim also has one when she walks up and startles him. Kim refers to Ron as weird, though she adds that "I like weird".
- This is the first time in the series where Monique and Wade meet in person. The two already knew each other, but this is the first time they met personally and seeing her in person is what causes his crush.
- This episode features the first interrupted kiss between Kim and Ron in Season Four, something which becomes a running gag in the season.
- Wade's father receives a first name: Lontaine. However his mother never did.
- Kim thinks nothing of zapping Monique herself to shut her up, and threatens to do it again latter.
- Kim works at her job at Club Banana in the episode but Ron's off work from Smarty Mart due to "Food Chain Issues" and cages being left open in the pet department, making it the only episode where Kim was working and Ron wasn't.
- The title is a play on “The Butterfly effect”.
- In this episode, Kim has her original mission outfit, which was destroyed in "Clothes Minded". However, she had her new mission outfit in "Clean Slate", which was stated to take place before this episode.
- In Paris, while Ron and Wade are hiding behind the bushes and discussing the plan, Monique has two heads.
- Wade claims that the batteries had lasted for about four years, but this runs contrary to the Kimmunicator being destroyed during "A Very Possible Christmas" during Ron's fight with Drakken on his Drak Force One spaceship. He sent Kim a new one shortly after.
- Ron refers to the other time that he wore a dress, but he was mind switched with Kim at the time. This is a reference to the episode "Mind Games".
- Ron uses Le Goop hair gel on Wade, which was first introduced to Ron by Francois in "The New Ron".