16岁的少女松前绪花（伊藤かな恵 配音）是一个自由生活在东京的普通女高中生。一如这座虽则繁华却死板枯燥的大都市，她的生活平淡无味。正处在幻想的年纪，绪花时刻期盼着“突然的夜奔、突然的爱恋、突然的离别”这类事情的发生。
对此感到焦躁的日本政府为了解决突然出现的Phenomenon而组织了特殊部队。总务省情报管理厅管辖扩张现实特殊战略队群特别攻击队——又名扩张少女系三重奏。
An affluent medical professor, Komiya, and his bossy wife, Tokio, are to look after Setsuko, their high-spirited niece from Osaka. Setsuko is a liberated woman who does what she wants, including smoking, even though she is a minor. On Saturday, the professor does not feel like going to his weekend golf game, but his wife packs him off anyway. So he leaves his bag at the apartment of his student Okada, and goes to a bar with a friend. Setsuko traces him there, and insists that he take her to a geisha house. When she gets rather tipsy, the professor calls Okada to take her home, while he sleeps at Okada's. The wife becomes suspicious of Setsuko when she sees Okada bringing her home, and also of her husband when she discovers that he did not go golfing.
