The Backyardigans find Atlantis: The Lost Empire

The Backyardigans find Atlantis: The Lost Empire is an upcoming Nick Jr./Disney crossover to be made by Hiatt Grey.

Plot
Many centuries ago, a tsunami threatens to drown the island of Atlantis. In the midst of an evacuation from the capital city, the Queen of Atlantis is caught by a strange, hypnotic blue light and lifted up into the "Heart of Atlantis", a powerful crystal protecting the city. The crystal consumes her and creates a dome barrier that protects the city's innermost district. She leaves behind her young daughter, Princess Kida, as the island sinks beneath the ocean. Her husband, King Kashekim Nedakh, is blinded during the event as he protects Kida from the light

In 1914, Milo Thatch, a cartographer and linguist at the Smithsonian Institution who is marginalized for his research on Atlantis, believes that he has found The Shepherd's Journal, an ancient manuscript that contains directions to the lost island. Though the museum board declines his proposal to search for the journal, a mysterious woman, Helga Sinclair, introduces Milo to Preston B. Whitmore, an eccentric millionaire. Whitmore has already funded a successful effort to retrieve the journal as repayment of a debt to Milo's grandfather, recruiting Milo to lead an expedition to Atlantis, as soon as he receives it. Whitmore also builds a mercenary task force 200 - strong, equipped with military grade weapons and munitions.

The expedition departs with a team of specialists led by Commander Lyle Rourke, who also led the journal recovery expedition. They set out in the Ulysses, a massive submarine. During the journey, they are attacked by the monstrous Leviathan, a robotic lobster-like creature that guards Atlantis' entrance. The Ulysses is subsequently destroyed, but Milo, Rourke, and part of the crew escape to a cavern, described in the journal as the entrance to Atlantis.

After traveling through a network of caves and a dormant volcano, the team reaches the borders of Atlantis, where they are greeted by Kida. However, Kida's father, the King of Atlantis, is not pleased with the team's visit, but agrees with Rourke for a one-night stay. Kida enlists Milo in deciphering the Atlantean written language, long forgotten by the natives. By diving deep within the city's submerged ruins and translating underwater murals, Milo helps Kida uncover the nature of the Heart of Atlantis: it supplies the Atlanteans with power and longevity through the crystals worn around their necks. He is surprised this is not mentioned in the journal but recalls that a page is missing.

Returning to the surface with Kida, Milo discovers Rourke has the missing page. Rourke and the crew betray Milo, intending to bring the crystal to the surface and sell it. Rourke mortally wounds the King while trying to extract information about the crystal's location, but finds it himself hidden beneath the King's throne room. The crystal detects a threat and merges with Kida, whom Rourke and the mercenaries lock in a crate, and then prepare to leave the city. Knowing that when the crystal is gone the Atlanteans will die, Milo berates the crew for betraying their consciences, and ultimately convinces them to leave Rourke and remain in Atlantis. The King explains to Milo that the crystal has developed a consciousness; it thrives on the collective emotions of the Atlanteans and will find a royal host when Atlantis is in danger. He then reveals that the sinking of Atlantis was caused when he attempted to use it as a weapon of war. As he dies, he gives his crystal to Milo, telling him to save Atlantis and Kida. Milo rallies the crew and Atlanteans to stop Rourke.

In the ensuing battle inside the volcano, Helga, and the accomplices die, while Rourke is killed after turning into a crystal monster by Milo after he had cut his arm with a broken shard of the crystal and being chopped up by the propeller. Milo and the others successfully fly the crystal back to the city, as the volcano erupts. With lava flowing towards the city, Kida (in her crystal form) rises into the air and creates a protective shield. The lava freezes and breaks away harmlessly, showing a restored Atlantis, and the crystal returns Kida to Milo. The surviving crew members return to the surface and promise to keep the discovery of Atlantis a secret. Having fallen in love with Kida, Milo becomes the new king of Atlantis, along with Kida as the new queen, and stays behind to help her rebuild the lost empire.