In the barren Sand Land, a human named Sheriff Rao decides to head out in search of the Legendary Spring to save his people who are dying of thirst. The Fiend Prince Beelzebub and the demon burglar, Thief, soon join him, but what awaits the unlikely trio are desert monsters and nefarious bandits.

The trio replenish their supplies at a nearby town when they come across a Royal Army tank. After taking the vehicle for themselves, a Royal Army plane discovers their theft and engages in battle. Consequently, the secrets that the Royal Army has kept hidden begin to be unveiled.

As Sheriff Rao's tragic past is revealed, he vows to continue his fight with Zeu and the Royal Army. Meanwhile, General Are seeks revenge as he leads his tank squad into battle with the trio who stole the Royal Army's tank.

As Rao tells General Are the truth behind the tragic explosion, the Swimmers suddenly make their move. Beelzebub takes off alone to capture them when he discovers something astonishing. Rao and Thief, hot on Beelzebub's tail, are shocked to see what The Fiend Prince has found.

Vowing to keep their discovery a secret, the trio turns the tank towards the Royal Army's water supply. As they learn the truth behind the King's water source, they face their final battle with Supreme Commander Zeu and his terrifying Insect Men.

An enraged Beelzebub overwhelms the Insect Men with his fiend powers while Rao and Thief infiltrate the Royal Army's mega aircraft carrier to stop Zeu. Rao and Zeu face off, but the cunning Zeu proves difficult to defeat. The trio's adventure reaches its climax in an epic battle with the Royal Army.

A few days after the river begins flowing again in Sand Land, Beelzebub leaves to deliver supplies to the Picchi. He meets an angel who calls himself a hero and a biker girl being chased by a drone. He then learns of a place called Forest Land. A new adventure unfolds for Beelzebub, Rao, and Thief.

Arriving in Forest Land, the trio is amazed by the lush scenery. When Ann takes her new friends to meet Lango, the leader of The Resistance, they learn King Jam will soon be executed. Surprised, the four rush to the capital in hopes of saving the king.

The half-cyborg Supreme Commander Bred attacks Rao and Thief, but Rao's quick wit helps them escape. Meanwhile, Muniel uses a cowardly trick to corner Beelzebub and resume King Jam's execution. However, Ann's mysterious power begins to reveal itself.

After hearing about Lango's past, the four realize where the final Aquanium is located. They rush to Demon Village, only to find King Lucifer facing off with Muniel. They are surprised to see Muniel holding an urn that looks just like the one the fiend, Lilith, had sealed away.

The long-awaited invasion of Sand Land's capital can now begin as Muniel uses the Aquanium to reactivate the Flying Fortress Garam. Determined to infiltrate and stop the flying fortress, the four seek help wherever they can get it.

Despite Bred's protests, Muniel refuses to halt his destructive plans. As Muniel steers Garam towards the capital, Beelzebub and his friends attempt to surpass Bred and infiltrate the fortress, knowing they will have to face Muniel.

The final battle between the devil and the angel. Behind the scenes, Aquatium, the source of Garam's power, had become abnormal and out of control. If Garam were to crash to the ground, there could be a huge explosion. As Beelzebub's battle draws to a close, An scrambles to stop Garam from crashing.