Plot summary

In this movie based on a book, Roger Walker plays with toys in a bathtub, with a patch over his left eye, but he is not taking a bath. His family is getting ready to leave.In 1935 in Portsmouth, England, the Walker family is getting on a train. The father is serving on a destroyer in the South China Sea, and the mother is left behind to take care of five children. Roger's age is not given but he is quite young, and John and Susan are older. Tatty is enthusiastic and has an active imagination, and she says several times that she is nine and a half. Occasionally she narrates the movie. Vicky is a baby.Before getting on the train, Roger drops a stuffed animal. A sinister looking man picks it up and hands it back to him, telling him to be more careful. A man with sunglasses is with him.On the train, the children open presents from their father. John gets a knife, which Roger is not old enough to have. Roger gets a spyglass. While their mother is smoking a cigarette, a rude man bumps into her and then joins the children in their compartment. He picks up Vicky and tells the children not to tell anyone he was there, threatening them if they do. Tatty thinks he is a pirate and later refers to him as Captain Flint. The man goes out the window and hangs onto the train. The men who Roger saw earlier are walking across the top of the train, and one has a gun. The man they are chasing jumps off the train.The family arrive at their destination and are picked up by Mr. Jackson, a kind man who takes them to the place they will stay. They expect to stay in the same place as last time. Mrs. Jackson welcomes the family but is not nearly as nice and constantly criticizes her husband. The kids (excluding Vicky, of course) don't even bother to change out of their nice clothes but go immediately to the top of a hill where they can see an island in a large lake. They want to go to the island and camp. Also on the lake is a houseboat that looks abandoned. John throws rocks at it and accidentally breaks a porthole. A man comes out, angry. It is the man who they met on the train. Nearby are the men who were chasing him, and they sometimes speak in Russian.Jim Turner, who is apparently a travel writer, goes to where the Walkers are staying and explains what happened. The kids' mother denies they did anything, but Jim has the rock John threw, with a pirate design, and John has to admit he did it, but it was an accident. He apologizes and his mother promises he will pay for the damage.Mr. Jackson shows the kids his sailboat called The Swallow. John and the kids are taught what to do to sail the boat, and how to set up tents. Roger is too young and gets upset when told he can't go with the others. However, once he is taught to swim, he is allowed to go. One last detail: permission is needed from the kids' father. They go to the post office, where they are told only a telegram will be fast enough. Also visiting the store where the post office is located are the suspicious men who were chasing Jim. They need a room and are told only one is available but it has two beds.Permission is granted, and the kids are off on their adventure. John is the captain, Susan is second in command, Tatty is assigned the ship's log, and Roger is the lookout. Before they arrive at the island, the basket containing their food falls overboard, and despite their best efforts, it sinks. The kids land on the island and declare it to be Walker Island, and they set up camp. The matches were with the food, so they can't start a fire. However, they see a fire off in the distance. Eventually, the cake Mrs. Jackson made washes up on shore in a container that kept it dry. Later, Susan and Roger worry when John and Tatty disappear, and they find a threatening sign which says Amazons do not want trespassers on their land. John and Tatty find two rough-looking men both named Billy, who know who Jim is and tell them to give Jim a message. They also teach the kids how to build a fire, and just in case they can't do it the traditional way, they give the kids matches. John visits Jim, who is not happy to see him. John reminds him he did not tell their mother where they had seen him before. Later, John visits when Jim is not on his boat. He sees a paszport and photos and documents that indicate he may be up to no good.The kids are reunited, and they see another sailboat out on the lake called The Amazon. Eventually, the occupants of the boat reveal themselves and tell the kids they are trespassing. They are Nancy and Peggy Blackett, and they say the island belongs to them.The Walker kids follow the Amazon to a carnival and while Roger is assigned to watch the boat, the kids join a parade where people are in fancy costumes. They also go to a store to get food. John let Roger have his knife, which was a mistake, because he loses it. The men chasing Jim arrive in a boat, which is where the knife ends up. Jim needs to send a telegram. The suspicious men manage to get the telegram before it goes out, and now they know who Jim's contact is.The two sets of children agree to fight a "war", with the winner being the one who captures the others' boat.Jim is Uncle Jim to the Blackett girls, who live with his sister, apparently the girls' mother. She is bothered by how much he travels.The kids realize they can travel at night and have an advantage. Tatty stays behind and, even though she is too young to light matches, is given special permission to do so to light a lantern. Roger falls in the lake and is rescued. The Blackett girls see the lantern and think they have won. The Walker kids capture the Amazon and the Blackett girls have to surrender. At this point they decide to be friends.Jim discovers someone has been on his boat and stole items, and he contacts the harbourmaster, who brings the kids' mother to the island. The kids are in trouble. John's knife was found on Jim's boat.Finally, Jim is visited by one of the men who are after him. They were apparently the ones who stole from Jim earlier. Jim denies his accusations, saying he is only a travel writer who likes Russia. The man insists Jim go with him, pulling out a gun. Then he knocks Jim out and the two men take him with them. They run into the kids, and John manages to get a gun and tries to force the men to let Jim go. The man tells John he doesn't have the courage to shoot him, and Jim says it takes more courage not to shoot, persuading John to just let them take him.The three men are about to leave on an airplane. But the kids tie ropes to the plane. This does not stop the plane from taking off, but it does give Jim time to get loose and overpower the men. The pilot is forced to land and the men are arrested.Jim plays with the kids, and while they all pretend to be pirates, he walks the plank.