Rainbow is out and about, trying to brag to her friends about some stunt she's just created, but they're all busy. Getting desperate, she eventually tries to brag to Spike, who isn't interested. She grows frustrated, eventually asking what kind of friend Spike is, when Spike claims that they are not friends. She is shocked to hear this, and asks why Spike thinks this.
Spike explains that he just assumed they weren't. They don't hang out, she never invites him to see her perform or to pull pranks on others (Cloudsdale Young Flyer's competition can be referenced) and they never talk to each other when they're all together. In Spike's opinion, he's friends with the other girls, and Rainbow's friends with Twilight (hence why Rainbow followed during the dragon migration, if the story is set after that episode). But Rainbow and Spike? Not friends.
Not pleased to hear this, Rainbow sets out to change it. The bulk of the story consists of Rainbow's various attempts to befriend Spike, as well as the other girl's reactions to Spike's opinion and Rainbow's actions. It can end several ways, with my personal favourite being Spike befriending Rainbow only after she gives up, and he realises how hard she was trying.
An alternative way of doing this story is for Twilight to discover Spike's opinion, and making it her mission to bring the two closer together. The other girls can either support her or try to talk her out of it. Meanwhile Rainbow and Spike are both stuck dealing with Twilight's antics, and end up inadvertently bonding over their attempts to get out of bonding.