Frances' brother, Sergeant William Schiller, develops a gyrocompass, a critical device capable of pinpointing The Tiger Shark's location.

When Sergeant Schiller is kidnapped by The Tiger Shark's henchmen, Grant and McGowan must race against time to rescue him and secure the gyrocompass.

Throughout 12 thrilling chapters, the Marines face various challenges, including traps, aerial attacks, and close-quarter combat with The Tiger Shark's operatives.

Each chapter builds on escalating stakes as the Marines strive to prevent the villain from disrupting their mission and uncover his true identity.

In the final confrontation, The Tiger Shark is unmasked and defeated, ensuring the Marines can complete their airstrip and maintain control over Halfway Island.