Angel Trainee Flonne is sent to the Netherworld to assassinate the Overlord, King Krichevskoy. Flonne spends two years searching for Krichevskoy, and eventually wakes his son, Prince Laharl, who doesn't beleive in love. After learning his father has died, Laharl sets out to claim his rightful title of Overlord as heir to the throne, and Flonne decides that she will stay in the netherworld even though her target is already dead to learn the true nature of demons... and possibly carry out the mission with a new target.

When Laharl demands a place to spend the night in the middle of the desert, he, Flonne, and Etna are guided by the Prinnies to the Netherworld Hall of Treasures. But when Vulcanus causes Flonne and Etna to switch personalities, chaos and hilarity ensue.

When the Prinnies go on strike, Laharl begins pillaging local castles in an attempt to get them some money. However, when his father's old butler, Zenisuky, poses as the Overlord and abuses his power to tax everyone, his son, Koganesky, hires Laharl to kill his father. Will Laharl kill Zenisuky, or will Koganesky have a change of heart?

Laharl and Etna stop at mansion after getting lost in a forest, except for Flonne, whom was ditched in the desert, where they encounter a woman named Sardia. Sardia offers food and hospitality to the two, but when she attacks them at dinner, it becomes apparent that she's an assassin after his life. Can Laharl defeat an assassin that has slain over 2,000 demons? Maybe... Maybe not!

Laharl, Flonne, and Etna arrive at the castle of Maderas, the demon who placed the bounty on Laharl's head. However, after he reveals that Etna is the one who poisoned him and tried to kill him, Laharl and Flonne become trapped in a board game full of embarrassing traps. When Maderas goes back on his promise to restore Etna's memories, Etna becomes to determined to find them for herself, but what about Laharl and Flonne?

Now free of assassins, Laharl, Flonne, and Etna stop for lunch at a saloon. The trio soon meets up with a strange girl named Maharl claiming to be Laharl's sister, but Laharl questions her relation to him. Could this strange girl really be Laharl's younger sister?

All the Prinnies suddenly vanish, and without anyone to do his chores, Laharl becomes bent on finding them. He, Flonne, and Etna manage to locate them and uncover a plot to take over the Netherworld. Will Laharl thwart their plans? And who is this strange pink Prinny that has appeared before the group?

Their spaceship finally repaired, the Earth Defense Force challenges Laharl to rematch. Laharl accepts Gordon's challenge on the condition that he takes him to his castle. As Laharl prepares to begin his newfound Overlord rule, the Netherworld is invaded by Earth without Gordon's prior knowledge. Will Laharl be able to stop this invasion?

Laharl and Gordon team up to rescue Jennifer, Etna, and Flonne from the Earth Defense Force, but Kurtis refuses to allow Gordon to do such a thing. After defeating him, Gordon proceeds to where Jennifer and her father are, and discovers that Jennifer had been "modified". To make matters worse, a bomb is set to destroy everyone on-board the ship. Is there any way to save the girls and make it off the ship in time?

On their way to Celestia, Laharl, Flonne, and Etna find themselves in a ice-filled land lit up by a red moon. When the Prinnies starts acting strange, the reason behind Laharl's hatred for love is finally revealed. And who is Big Sis Prinny?

Having finally arrived in Celestia, our heroes discover that Vulcanus was responsible for the attack on the Netherworld. Upon meeting with Seraph Lammington, the trio learns why Flonne was sent to kill Laharl's father and wake Laharl. However, despite the Seraph's kindness, Flonne is still punished for her sins of hurting Angelic Soldiers. In order to save her life, Laharl performs an act of love so amazing that it is viewed by not only all of Celestia, but Earth and the Netherworld as well!