Bart and Joe live on the east coast of Florida and have been best friends for years. Bart is recently married with a new, high-pressure job at Kennedy Space Center and a baby on the way. He and his wife, Mary, are doing their best to navigate the choppy waters of new marriage, adult responsibility, and impending parenthood. Meanwhile, Joe is still working construction and picking up girls on the beach in his restored 1967 VW van.

But despite their differences in lifestyle, Bart and Joe remain close, mainly due to their shared love for SCUBA diving. With plans to go cave diving in one of Florida's many natural springs, they drive to Blue Grotto State Park in the central part of the state, bringing along Mary and Joe's new girlfriend, Jill.

Arriving at the park, they find they have the place to themselves. They set up a picnic alongside the spring, and Bart and Joe decide to get right into the dive. It's a dangerous new adventure, and Mary is concerned for their safety. But Bart and Joe are confident and experienced divers. As the women remain on the surface, the men tie a lifeline to a submerged tree and plunge in. Battling against the strong upsurge of water, they work their way into the sunken cavern. Once there, Joe's light gives out. Instead of returning to the surface, they decide to rely on Bart's light and press on to explore the Well, a deep tunnel that presumably connects to other underwater caverns across the state.

Once in the Well, however, things go wrong. A decision must be made by one of the divers. Does he save himself, or does he help his best friend, his diving buddy?