Season 1 of Fukubiki! Triangle premiered on April 30, 2010.

Having finally gotten together with Miharu, Ushio leads a healthy daily sex life. That comes to an end one day when Miharu takes a bad fall on the way to school and is hospitalised.

Together with Miharu's sister, Futaba, Ushio keeps vigil at the hospital until Miharu finally awakens. However, she does so without any memory of Ushio! Not only that, she is no longer the cheerful and positive Miharu that Ushio is familiar with. Instead, this Miharu is dour and moody. Futaba reveals that this was the Miharu of the past, before she met Ushio.

While Ushio was worrying about what to do, he becomes aware of Futaba's heavy breathing, flushed cheeks, and the lust in her upturned eyes. Despite his worry about Miharu, his will crumbles before Futaba's seduction.

Season 2 of Fukubiki! Triangle premiered on March 27, 2015.

Ushio's love affair with Miharu's sister Futaba isn't as hot and active as Futaba would like. She decides to take a more proactive route to heat things up, attempting to seduce him, tying him up, drugging him, and so on. However, Futaba's antics might be too strong for Ushio's liking and end up backfiring.