An adventurous boy named Catch Kandy believes he has accidentally killed his uncle, Earle Kandy. He and his sister Kate then run away into bushland near Taronga Park Zoo. They meet a zoologist called Christian Faber who is willing to help them.
Catch and Kate, who have taken refuge in the bush land across the Sydney harbor near their uncle's home, befriend a zoologist called Christian Faber. The man becomes like a big brother to them. Meanwhile, to safeguard his privacy, the kids' uncle Earle Kandy, decides not to raise the alarm on the kids' disappearance. Christian then drives Catch and Kate to Roselands shopping centre so they buy new clothes to avoid being recognised.
Some ex-college friends of Christian give him a bashing when they finally lay hands on him. Christian decides not to report the matter to the police. Meanwhile, Catch and Kate's guardian uncle is getting worried about his missing wards.
After Catch and Kate play at Canon Park, Kate is later approached by a confused elderly man who believes he is still in a war battle. After taking the sick man to Christian's cave, he collapses. Munganye arrives to help take him back (with police assistance) to a nursing facility.
Catch decides to board a fishing vessel with a small group of Italian fishermen. As another rival fishing vessel is about to crash into them, Catch falls into the water and is eventually rescued. Meanwhile, another fisherman recognises Kate from a circulated missing persons newspaper portrait and, as there is a reward for her, he forcibly takes her to the police at Taronga Zoo. Through Christian's help she is set free.
Kate follows a street marching military band. She marches behind them into Victoria Barracks where she is questioned until the police arrive. Meanwhile, after unsuccessful attempts by Catch to enter the barracks, he finally enters and both he and Kate escape by hiding in a laundry van which leaves the barracks.
Catch catches a youth robbing a store. Catch decides not to report the matter to the police so the youth urges Catch to accompany him in flight.
Earle invites Det. Sgt. Rourke to dinner beside Rose Bay Flying Boat Aeroplane Service to discuss the kids' whereabouts. Meanwhile, Munganye boards an aeroplane to Lord Howe island unaware there are two crooks on board. Christian, in his small boat, stops them taking off enabling the crooks' capture. The kids narrowly avoid being seen.
Christian promises to take Kate and Catch swimming but at the last minute he has to cancel out, so he leaves them with Slow Kelly. Unfortunately, Kelly is involved in an accident and has to be hospitalized. Meanwhile, a guy recognizes Catch from the "missing" poster that Catch's uncle has been circulating.
Catch has been asked to take care of a boat until a police case over ownership is settled. In the meantime, Catch goes for a ride in it, but when it capsizes, Slow has to go to Christian for help.
Catch and Kate befriend another runaway kid. They bring him to meet Christian but the two fail to get along. When the lad misbehaves again later, Christian is compelled to ask him to vamoose. Enraged, the fellow threatens to blow the whistle on Catch and Kate.
A diver gets washed up on the beach where Catch and Kate are playing. Not knowing that the man had been hired by a syndicate to steal diamonds, Catch rushes to get medical help for him. In his absence, two syndicate men arrive on the scene. Seeing Kate there, they decide to capture her to prevent her from blowing the whistle on their activities.
Earle, Mrs Wayne and a friend visit Taronga Zoo. Mrs Wayne is spotted by Catch and Kate. When Mrs Wayne informs Earle that she believes she saw Catch, they go on a search for the kids. After a fruitless search, Earle finally and unexpectedly sees Catch and Kate. The kids are startled and speechless to see their uncle still alive. They are finally and happily reunited.