The spaceship piloted by Oscar, a friendly alien, crashes in Tommy's backyard and the two immediately strike up a friendship. Tommy introduces Oscar to his circle of friends: Professor Leonard, an eccentric scientist; Yucari, a formidable female martial arts practitioner and Peter, an aspiring musician. Yucari discovers Cesare, a villainous businessman, plans to turn the city park into a desert by killing all the plants with his special herbicide, then buying the property for next to nothing and turning it into a parking lot. Professor Leonard learns when music is played, the effects of the herbicide are spectacularly reversed, but when the crew is trapped in an abandoned tunnel, they may not be able to save the park before the mayor sells Caesar the deed.
Scientists at SETI alert the army that an alien spaceship has landed in Tommy's backyard and soldiers are dispatched to find him. Before they meet Oscar, the little alien is kidnapped by Cesare and his goons because the crooks think they can sell him to the government for oodles of cash. Oscar proves to be a hard alien to keep down as he innocently creates mayhem on board Cesare's massive boat until Tommy and Yukari can rescue him.
When Professor Leonard receives an urgent request to come to the aid of an old friend working with Africa, he leaves a box that came with the message in Tommy's care. Shortly thereafter, the globe-shaped object hatches and a baby dinosaur emerges. Tommy is determined to return the young dinosaur to his mother and follows Leonard into the jungle where he learns that Cesar plans to dam a river, depriving the locals of water they need to survive. He's also captured the mother dinosaur to serve as the prime attraction for his new theme park. Tommy and Oscar attempt to frustrate Cesar's schemes and unite the mother and baby dinosaurs again.
Professor Leonard has invented a device to make animals intelligent. However, in the midst of his trial at a circus, all the animals were sucked by an enormous flying ark and a split second later, the spacecraft disappeared in the dark sky. Tommy and Oscar soon discover that their old enemy Cesar is capturing animals to use at his new resort where patrons can shoot the beasts - for a hefty fee, of course.
The whole town has gone crazy over an upcoming exhibition basketball game between the Rainbow Team, a local team and the renowned Black Cloud squad and the arena is sold out. Minutes before the contest, all the players from Rainbow Team have a strange stomach illness and are sent to the hospital. The Rainbow's JV team, consisting of Tommy, Oscar, Peter, Yukari and the wheelchair-bound Charley agree to play in their place.
Professor Leonard invented the Dispose-a-bin, a garbage can that transports trash into a black hole in the middle of the galaxy. His invention won him a all-expense paid holiday in Egypt for four people, so he brought Tommy, Oscar and Yukari along. While touring the ancient ruins, the gang meets their old enemy Caesar who has converted an abandoned lighthouse into an interstellar rocket. Yukari discovers his evil plan, but is captured and tied to the missile. Can Tommy, Oscar and the Professor rescue her before she's shot into space?
Tommy, Yukari and Peter encourage Professor Leonard to build a larger prototype of his machine that could harvest clouds to deliver rain to countries parched by drought. Leonard needs a much larger space to build the device. The kids discover that, when they pool their piggy bank money, they can afford to buy a rundown castle with plenty of indoor space. The castle is reputed to have a huge treasure hidden inside its walls, which attracts the interest of Cesare and his goons.