In Super Paper Mario, Mario collects eight Pure Hearts to stop Count Bleck.
The Pure Hearts are magical artifacts central to the game's plot. Merlon sends Mario on a journey across the dimensions to recover all eight, because their power is needed to oppose the Chaos Heart and weaken Count Bleck's apocalyptic plan.
Mario does not complete the quest alone. Peach, Bowser, Luigi, and several allies eventually join him, while Bleck's minions—including Dimentio, Mimi, and O'Chunks—try to stop the heroes. After the eight Pure Hearts are gathered, the group confronts Bleck at Castle Bleck.
A common mix-up is confusing the Pure Hearts with the game's four playable heroes or with the eight chapters of the adventure. The number is eight, and the Hearts remain important even after Bleck is defeated because the final conflict over the Chaos Heart continues.