When Jack opens it, he finds a couple of large beans. After staring at them a while, he throws them into a field. The next day he discovers a bean stalk which is the talk of the town. But he discovers this is no ordinary beanstalk when a tendril grabs him, and he and the beanstalk shoot up into the clouds. He awakens to the sight of dinosaurs and a flying castle. It sounds so much better than it is.
The beanstalks are finicky. Only the person who planted them can climb them. Plus they disintegrate within a few hours, something that Jacks' father should have told him when Jack discovers him in the flying castle. Dad is the same age as when he left.
Okay, the flying castle is kind of neat, but why does it run on coal and how does it even move based on the minuscule amount of coal that powers the forge?
And then there's the matter of the beans. I could have sworn Jack received two beans and threw them both into the field. But later the girl with him (not sure if it's his sister or friend) plants a bean, and at the end we see Jack has another one. Are they asexually reproducing, self replicating beans, or is this an oversight? And if Dad is leaving his son beans, how about a note? It may have been nice to give Jack a clue as to why you're giving them to him.
As for the rest, the police are right out of a Monty Python sketch. The robot suit Jack builds doesn't protect his head. Who wants to fight dinosaurs in a metal suit that exposes your cranium? And what time period does this take place in? There's WWI army uniforms but Jack's motorcycle isn't that old and neither is his leather jacket.
And why is Jack building a robot in his barn? Well, it's robotic but he gets inside it, and guides it with his overly exposed skull, ready for a proper crushing. Also does Jack kill any giants in this movie? No. Are there any giants in this movie? No. We're left with dinosaurs and an evil lady who wants to rule the planet with her dinosaur army. Bah!