Some days later, Tom and Stella sit on a grassy mound on “The Island,” a cluster of trees in the garden’s center. Harry has been missing for almost a week, and Mrs. Moon is distraught. Stella twirls her friendship bracelet from her Hong Kong friend Hannah while Tom picks at the mound with his trowel. Despite Stella’s warning about Charlie, Tom digs until he uncovers what appears to be a rounded wooden lid, which he believes is a treasure chest. As he continues to dig, Stella realizes that it’s not a box but a boat.

Rustling bushes startle them, but instead of Charlie, Harry emerges from the undergrowth, soaking wet. He flees toward home when he sees them. Their mother calls them to leave for their grandmother’s house. Stella insists that they cover the excavation with a log to hide it from Charlie. Tom is frustrated at having to conceal his discovery. Stella theorizes that there must be water nearby because of the boat and suggests that Harry knows where it is.

That night, Stella has trouble sleeping, thinking about the boat and the wet dog. At five o’clock in the morning, a clanking sound wakes her. She looks out her window into the misty garden and sees Harry trotting toward The Island. She wakes Tom, who is dreaming about a mole attacking him. They sneak outside in their pajamas and race to The Island, but Harry is gone, and the log remains undisturbed.

While walking back across the damp lawn, Tom thinks he sees three or four moles scampering in a circle, but they vanish when he blinks. He dismisses it as a trick of the light. Later that morning, the telephone rings—it is Mrs. Moon, still distraught over Harry’s disappearance. The call confirms for Tom and Stella that their early-morning experience was real, and they resolve to search for Harry after breakfast.

At 10 o’clock, Tom and Stella return to the mound with picnic lunches, a torch, and binoculars. Stella crawls into the rhododendron bush from which Harry had emerged. Tom scans the garden and again thinks he sees moles, but they disappear. Following Stella into the thicket, Tom’s arm slips into a deep, wide hole. They discover a rusty metal ladder fixed to the earthen wall, leading down into darkness.

Stella descends first, holding her torch between her teeth, followed by Tom. At the bottom of the ladder, the tunnel wall becomes tree bark with branches and nodules providing handholds. As they climb lower, daylight appears from below. They emerge from the base of an enormous tree onto sun-drenched grass beside a beautiful lake surrounded by crimson flowering bushes. Tom spots a boy in ragged brown clothing rowing frantically toward them. The boy lands and has them help pull his boat ashore under the tree. He warns them not to tell anyone before fleeing into the woods. Stella thinks she sees moles across the lake, but Tom dismisses this. They decide to row across the lake.

Tom and Stella row across the lake and pull the boat ashore. Upon entering a small wood, they hear children’s voices. They peer through a bush and see two young girls in elaborate purple dresses and a woman in old-fashioned clothing sitting on the lawn. Tom shouts when he spots Harry running to the children, revealing their presence. Harry rushes to them, and Stella finds herself face-to-face with the two girls, Sophie Gladstone and Emma Gladstone, who stare at their modern clothing in astonishment.

The girls’ governess, Miss Walker, appears, surprised to see Stella wearing trousers. Emma, the younger and friendlier sister, introduces herself as 10 years old and her sister, Sophie, as 12. Stella introduces herself as 11 and Tom as eight. Stella notices that her friendship bracelet is missing from her wrist. Emma guesses that they’re the Australian cousins of her best friend, Lucy Cuthbertson. To avoid a difficult explanation, Stella confirms Emma’s guess, silencing Tom with a fierce look.

Miss Walker invites Tom and Stella for a cool drink. As they help move a rug, Tom points to a nearby house and declares that it’s theirs. Sophie retorts that it’s the Gladstones’ house. Stella quickly covers, reminding Tom that their house is in Australia. A constable and a man named Crawley run into the garden, followed by Mrs. Gladstone, the children’s elegant mother. The constable reports that silver and Mrs. Gladstone’s coin purse have been stolen. Crawley says that he saw a young “ruffian” in the garden and eyes Tom suspiciously.

The constable asks the children if they saw a scruffy boy in brown clothes. Stella pokes Tom to keep him quiet, and all the children shake their heads. Mrs. Gladstone orders her children and Miss Walker inside, citing the danger of a thief on the grounds. She dismisses Tom and Stella, telling them to return to Lucy’s house and get proper clothes before marching away with her daughters and their governess.

Once alone, Tom angrily confronts Stella about her lie. Stella explains her theory that the tunnel took them back in time, reasoning that the buried boat in their garden came from this era and that the lake dried up in the future. Tom accepts the explanation and becomes excited about time travel. They row back across the lake and climb the tree, but the tunnel entrance has vanished. Climbing back down, they find the boy from the boat waiting for them.

The boy offers Tom bread. When Stella calls him a thief, he denies stealing and introduces himself as Jack. He explains that his father, Jacob, a builder who worked on the houses, was falsely accused of theft by a servant and jailed. Jack was in the house trying to retrieve his father’s tools from the cellar when he saw Crawley stealing the silver. Jack reveals that he escaped his pursuers when he saw the moles his father had told him about—the pursuers seemed to lose sight of him right after. Tom and Stella realize that they also saw moles just before finding the tunnel. Stella explains that they’re from the future and theorizes that the moles control the time tunnel’s appearance. After Stella reveals that the tunnel has disappeared and worries that they’re trapped, Jack reassures them that it will return and says he will wait with them. Stella shows Jack her iPhone, playing music that amazes him.

Over two hours later, with Tom asleep and the tunnel still absent, Jack suggests that they go to the Gladstone house for supplies. He produces a key to the kitchen’s back door that his father had kept. At 1:30 am, they row across the moonlit lake. Jack is fascinated by Stella’s torch, which she promises he can keep. They enter a large, old-fashioned kitchen and gather food from the walk-in pantry: cake, cheese, bread, and fruit.

Tom refuses to leave without seeing his future bedroom. Stella points them to the correct door, which, in this time, opens onto a grand dining room with a vast mahogany table, velvet chairs, a chandelier, and family portraits. They identify a portrait of Emma with Harry. Hearing footsteps approaching, Jack makes them hide inside the large fireplace chimney, clinging to a ledge inside the flue. While waiting, Jack points out his father’s mason’s mark in the brickwork. Tom uses the kitchen key to engrave a letter “T” beside it. When Tom’s foot slips and he crashes into the fireplace, he lands at the feet of Emma, who stands holding a candle. Tom reveals that Stella and Jack are with him. Emma recognizes Jack as the boy thief, but Stella quickly interjects to defend him.

After hearing Stella’s explanation of what has happened, Emma squeals with excitement, revealing that she knew the moles were special and had seen them herself dancing in circles at night. She accepts Jack’s innocence and helps them sneak outside. Emma confirms that Harry disappears all the time. Jack concludes that the moles appear only for special people. Just then, they see four moles scuttling in a circle on the lawn before vanishing, which Jack takes as a sign that the time tunnel is ready for them. Emma decides that she wants to come with them to the future.

Suddenly, a door slams. They hear Emma’s father, Mr. Henry Gladstone, and her mother calling for Emma and searching the garden. Emma is terrified and urges the others to flee. Mr. Gladstone spots Jack and gives chase. Jack creates a diversion, allowing Stella and Tom to escape. Emma tearfully tells Stella to find the tunnel and come visit again. As Stella and Tom run through the woods, they hear a child’s pained scream—Jack has been caught. Tom wants to go back, but Stella, crying, forces him onward, arguing that it’s too dangerous. They row back across the misty lake in guilty silence. Upon climbing the tree, they find that the ladder and tunnel have returned. As Tom ascends toward home, he silently resolves to return and rescue Jack.