A catastrophic undersea earthquake capsizes the luxury ocean liner Poseidon, forcing a small group of survivors to navigate the inverted wreckage to reach the hull. The protagonist and his companions must ascend through the ship's interior, which has become a labyrinth of twisted metal, rising water, and shifting debris. They face physical exhaustion, the threat of drowning, and the psychological volatility of other passengers who refuse to cooperate. The narrative maintains a high-stakes pace, focusing on the logical progression of their climb through the ship's decks while the vessel slowly succumbs to the ocean.

Readers frequently highlight the visceral atmosphere and the technical detail Gallico employs to depict the ship's destruction. Discussion often centers on the stark contrast between the characters' previous luxury and their current desperate circumstances. Critics note that the pacing remains consistent, effectively building tension as the group encounters new obstacles within the wreckage. The book is often cited for its influence on the disaster genre, providing a blueprint for survival narratives that prioritize mechanical realism over character melodrama. Many readers appreciate the focus on the group's collective effort to overcome the physical constraints of the sinking vessel.