Hu Bayi and his friends, after retiring from the underworld, help their friend Ming Shu to salvage pearls in the South China Sea. Unexpectedly, they encounter a shipwreck and fall to the bottom of the sea, but accidentally discover the remains of a thousand-year-old ancient country sunk in the sea. They experience many hardships in the seabed, and finally unite to defeat the deep-sea monster, and cleverly use the mechanical devices in the ruins to escape.