Dr. Maya Calder, a marine biologist in Havenport, Maine, discovers an unsettling phenomenon: crabs are dying without visible cause. Initially dismissed by local officials focused on tourism, Maya's concerns escalate as more marine life exhibits similar symptoms. She teams up with Ben Carrick, an old fisherman whose generational knowledge of the sea aligns with her scientific observations of the unusual scarcity of sand eels and lethargic lobsters.

After a flounder dies with signs of suffocation despite normal oxygen levels, Maya sends samples to her former mentor, Dr. Aris Thorne. Thorne identifies a highly potent, undetectable Cold War neurotoxin codenamed "Silent Tide," designed to mimic natural causes of death. He reveals it's an old leak, not a new one, prompting Maya to suspect a sunken military vessel in the treacherous "Graveyard" channel.

With the help of journalist Mark Jensen, who provides side-scan sonar equipment, Maya and Ben locate a military wreck with a visible rupture and a faint plume. They learn that an impending storm will push the toxin directly into Havenport's vital salt marshes and clam flats. Despite Navy assurances of a response team, their estimated arrival time is too late.

Determined to act, Maya, Ben, and Mark embark on a perilous mission during the storm to shift the sunken wreck using Ben's fishing boat, hoping to slow the leak and change its direction. They succeed in significantly reducing the leak just as the Navy cutter arrives. Months later, the leak is sealed, but the bay is scarred. However, signs of recovery, like a tiny green crab and diving gulls, offer a fragile hope for Havenport's healing.