In Florida, ten-year-old David has his telescope ready for the lunar eclipse, but his mother sends him to bed. When the eclipse finally happens, something mysterious goes on in the sky, making the ocean dolphins go crazy. Suddenly, a beam of light appears, and the dolphins swim around in excitement.
The light keeps moving as if searching for something and stops at some underwater ruins. The next morning, David’s grandparents Ben and Mary are taking care of the groceries with their friends from the retirement community they live in. It’s clear all these people are extremely close friends and make their days brighter by keeping each other company, although their age also means there are often deaths in the group.
Ben has a little secret that he keeps from his wife. Together with Joe and Art, he’s been sneaking out of the retirement home and crossing an old gate to break into an empty vacation house that has a lovely swimming pool. They spend a lot of time swimming and just enjoying some peace and quiet.
Meanwhile boat tour guide Jack gets some grumpy customers that only pay him half his fee for the trip. This isn’t enough to pay for his space at the harbor, so he may lose it soon. At that moment a group of peculiar tourists shows up and says they want to rent Jack’s boat for a whole month.
Jack immediately says yes and the group begins inspecting the boat very closely to make sure it’s what they need. They already have a map they want to follow, so they only need Jack to sail the boat and not worry about the tour. In the afternoon David comes to visit his grandparents and Ben decides to show him the swimming pool.
However when they’re about to reach the house, they notice Jack’s new clients renting the vacation house for the whole month. They claim to be cousins on a family vacation and they don’t care if the house is in a bad state. Their priority is a working swimming pool although they don’t want a pool man coming over.
Ben’s group has no choice but to go back. Later Ben goes to renew his driving license but he fails the eye exam, so now he can’t drive anymore. While putting the car away, David admits that he likes spending time with Ben and the other elderly more than people his age.
When the cousins finally get on the boat, they show Jack an underwater configuration map that is heat-sensitive. Jack has heard of these maps before and manages to follow this one correctly to find the spot the group wants to visit. The cousins put on their own scuba gear and dive in.
On the ocean floor, they use the advanced tools they’ve brought to retrieve some huge cocoons from the old ruins while the dolphins watch them closely. The cocoons are taken to the boat and the cousins put them in protective bags while Jack is amazed by the dolphins getting so close to people. When the boat returns to the harbor, Ben and his friends watch them from afar with binoculars and wonder if those big bags have illegal substances.
They also realize that if the cousins will be gone for most of the day, they can still use the pool. The next day the three men sneak into the pool house as usual and are surprised to find the cocoons in the water. They poke them with a tool and theorize they’re capsules hiding the illegal substances.
The trio decides to ignore them and swim anyway. After just a few minutes in the pool, the men feel invigorated and start swimming with an amount of energy they didn’t think they had anymore. They jump around and play games, yet they never get tired.
On their way back, they still feel very energetic and realize their wood is awakening. That night, Ben and Joe get dirty with their wives, and widower Art finally is brave enough to flirt with Bess. The next day the women are in a better mood than usual and the three men are excited to return to the pool.
The cousins are still diving to look for more cocoons and while Jack waits, he pokes at their strange machine only to get shocked. His poking triggers an alarm that thankfully ends soon and a dolphin laughs at Jack’s shenanigans. When Jack isn’t looking, the machine glows and gets the dolphin’s attention.
That night Jack tries to take a look inside the bags with the new rescues, but Kitty catches him red-handed and stops him. To satisfy his curiosity, she tells him they’re rescuing giant sea snails to take them to a museum because they’re very rare. While moving some equipment, Jack accidentally hurts his foot and panics.
Kitty quickly gives him a foot massage to help him deal with the pain and Jack uses the chance to flirt with her. However Kitty turns him down, saying she isn’t like the women he’s used to. After she goes to her room, Jack decides to spy on her and watches her take off her clothes.
Suddenly there’s a bright light and Jack is shocked to see Kitty taking off her human disguise, revealing she’s actually an alien. At that moment Kitty notices him and Jack panics, trying to run away. Kitty and the others immediately surround him, so he keeps them away with his welder.
Next Jack tries to start the boat to no avail. When Kitty gets closer to calm him down, he notices her skin falling and Walter fixes it before revealing he’s wearing fake skin too. Jack panics again and jumps into the water, where he’s startled by the laughing dolphin.
The group helps him get back on the boat and promises they won’t hurt him, in fact they’ll still pay for the service. These people aren’t cousins, they’re aliens known as Antareans. Thousands of years ago they set up an outpost on Earth that became Atlantis, the legendary continent that sunk to the ocean floor.
This tragedy forced them to go home but they left a few members of their species sleeping in those cocoons, so they’ve returned to rescue them. Jack understands that the aliens have no ill intentions and agrees to keep helping them. Meanwhile Joe goes to the doctor and is surprised to hear his cancer is gone.
To celebrate, the three friends take their ladies to a party and they spend the whole night dancing. The next day the trio invites Bernie to come to the pool with them, but when Bernie sees the cocoons he refuses to get in the water. At that moment the aliens come back to the house, so the men have to run to hide in the closet.
They watch how more cocoons are dropped in the pool and how Walter removes fake skin from his hand to activate the glow, which causes the cocoons to get bright as well. The rest of the group also takes off their costumes and one of them flies across the room to open the closet, finding the old men. Panicking, Ben and his friends immediately run back to the retirement home and ask the employees to call the police, but they don’t believe the story.
Joe’s wife Alma yells at them until they call the authorities, but when a cop arrives at the vacation house, Walter blames it all on the senile minds of old people and promises he won’t press charges as long as the men don’t break in again. For the next few days, the men feel sad and bored because they’ve lost the pool. To make matters worse, Joe’s cancer returns.
Ben visits Walter and explains the situation, swearing that they only want to swim and won’t touch the cocoons. Touched by Ben’s determination to save his friend, Walter gives them permission to use the pool. That afternoon the men invite their wives to come to the pool too.
However Bernie refuses to go anywhere near the aliens and doesn’t let his wife go there. The rest of the group has lots of fun in the water and soon they’re regaining their energy. When the aliens come back from diving, they casually say hi as if they were old friends.
The water heals Ben’s sight and he renews his driving license. Now the group can go out at night and visit various modern bars all over town, where Joe surprises them with his dance moves. They play games on the beach, join more of the retirement home’s activities, and have fun even when they're shopping.
Mary can climb trees with David and the aliens even agree to play cards with them. Sometimes the other elderly residents ask them about their newfound youth, but the group always keeps the secret. In the meantime Jack and Kitty spend a lot of time together and one night they go swimming with the dolphins.
Jack wants to get dirty with her but her anatomy is different, so she shows him how her species shows affection. Both of them get in the pool with no clothes and Kitty starts glowing. She gathers this glow outside her body and after it bounces all over the room it enters Jack’s body through his chest.
This gives him a pleasant feeling similar to the climax of getting naughty. A few nights later the rejuvenated group wins a bowling tournament and Ben worries their showing off may get people's attention. Everyone goes home except for Joe, who picks up a woman at the ice cream parlor.
It’s revealed they’ve been having an affair and Alma finally catches on. When Joe returns to the retirement home, he sees Alma leaving and tries to convince her to stay. Alma refuses to listen to him and after a short argument, she moves to Bess’ place.
The next day during lunch Joe takes out his frustration on Bernie, telling him he would bring his wife to the pool if he really loved her. An argument ensues and everyone in the room hears Bernie mention a “mountain of youth”. Bernie tells Joe this attitude is why Alma left him and Joe gets so angry he tries to punch him, but a security guard cuts in and gets punched instead.
When a second guard comes to help Joe begins to beat him up too and doesn’t stop until Art pulls him away. After hearing the argument, the other seniors realize the secret is in the pool in the vacation house. A large group of old people leave the retirement home and bring down the old fence to rush into the pool, where they start swimming and splashing without being careful around the cocoons.
They even use one as a toy. At that moment Walter comes in and yells to kick them out. When nobody reacts, he threatens to call the police, which finally makes them all leave.
Afterward the aliens open the cocoons and discover that the old people used all of the pool’s healing energy, so the aliens in the cocoons are now all shriveled and almost dead. Walter announces they won’t be able to help them and is surprised to notice his human disguise cry. Meanwhile Bernie brings some medicine for his wife only to discover she’s died.
The grief hits him so hard that he picks up the body and takes her to the pool to try to revive her, but since the water lost its power it doesn’t work. Walter finds him and apologizes for being unable to help. Minutes later an ambulance arrives to take the body and Ben decides to visit Walter to apologize in the name of his community.
Walter explains he never felt grief before because his species is immortal, yet now there are two dead aliens in the cocoons. He wants to put the rest of the cocoons back in the ocean so they can sleep again until he can come back with more power, but there’s no time because he and his crew must fly home the next day. Determined to make up for his mistake, Ben gathers a bunch of his friends and together they put the cocoons in the boat.
Jack takes them to the right spot and the seniors are given diving equipment so they can help take the cocoons back to the Atlantis ruins too. The cocoons are well hidden while the dolphins swim around them. Once the whole process is done, Walter points out that since he couldn’t rescue his friends, there’s extra room in the spaceship.
If the old humans want, they can leave Earth and start a new eternal life on the alien planet. The seniors agree but first they have to prepare a few things. Ben tells David he and Mary are leaving and David won’t be able to visit them ever again.
When David asks why and Ben mentions outer space, the boy doesn’t believe him. Mary hugs her daughter Susan and David one last time as a final goodbye, but she doesn’t cry until they’re far enough. Art takes all his savings from the bank and starts giving them away to random people on the street; then he gets married to Bess.
Joe visits Alma to tell her about the aliens’ offer. He explains he wants to go, but if Alma doesn’t, then he’d rather stay here with cancer to wait for the end with her. Feeling bad for him, Alma agrees to go.
The rest of the residents are also told about the offer and most of them agree to go. They wait for the employees to fall asleep and sneak out of the building without making any noise. While they board the boat, Bernie shows up to say goodbye because he’s chosen to stay.
Unfortunately Jack can’t start the boat so they’ll have to wait for him to do some repairs. At the same time Susan notices David is crying while waiting for another eclipse and wonders if he knows why her parents were behaving weirdly, but he refuses to tell. A worried Susan decides to visit the retirement home and when the security guard goes looking for Ben and Mary, he discovers all the beds are empty.
He immediately calls the police who starts searching the area with Susan’s help. However David sees the boat finally taking off and runs down the harbor, jumping at the last second to join his grandparents. The cops think it’s a kidnapping and alert the Coast Guard.
Soon choppers and more boats are going after the group. Jack pushes his boat to go faster and a chase begins as the cops threaten to come aboard. Realizing he’s putting his grandparents in danger, David gives them a last hug before jumping into the water.
The Coast Guard concentrates on saving the kid and Jack’s boat gets to leave without issues. When the eclipse finally begins, a spaceship appears in the sky and creates a thick fog to keep the Coast Guard at bay. In the middle of this fog the spaceship turns on a bright light to find Jack’s ship.
Walter pays him way more than they agreed so he can buy a new boat and thanks him for being a good friend. Jack hugs Walter and kisses Kitty, who promises to visit the next time they come to Earth. At that moment the spaceship’s light beam begins lifting the boat, so Jack jumps into the water and leaves in a life raft.
With the fog covering their escape, the aliens and the elderly are taken into the big spaceship, which quickly flies off into space. The next morning a funeral is held for all the old people that supposedly were lost at sea. Susan cries for her parents but David looks at the sky, finally believing Ben’s story.
