On a sunny afternoon Paul and his friends, the cat Kira and the dog Hugo, meet at the playground and play a funny guessing game. "I spy with my little eye, and it's... blue!" Paul calls. Kira and Hugo look around and merrily shout guesses: "The sky?" — "The slide?" — "That car over there?" — "The sea!!!" a voice cries above their heads. "I've seen it often from the air." Paul and his friends look up in surprise. It's the parachuting frog, who is making one of his daring jumps again and lands with a flourish in the sandbox. Then he folds up his parachute and hurries off.
Kira and Hugo keep guessing for a while what Paul could have meant, but they just can't find the answer. "The last carriage of Uncle Egon's train is blue!" Paul finally cries. "Look, the engine is over there at the station!" "Oh, that really was difficult!" complains Kira. "Tell me, have you ever seen the sea? I haven't," says Hugo. Paul and Kira shake their heads. "How about we ask your uncle if he'll take us to the sea sometime?" Hugo suggests. Paul and Kira are immediately thrilled by the idea. Paul's uncle is very kind and is happy to take them on a day trip to the sea — however, they still have to ask their parents for permission. They're supposed to leave tomorrow.
Paul can hardly sleep from excitement because he can hardly wait to finally see the sea! During the night, however, Hugo unfortunately comes down with a sore throat and a fever and cannot accompany Paul and Kira. "Have fun at the sea!" calls Paul's mum. "We'll pick you up here again this evening." The locomotive toots loudly in farewell, sets off, and soon Kullerbü station lies behind them.
Paul and Kira let the wind blow in their faces and enjoy the beautiful ride — past brown fields and green meadows where plump cows graze. After a while, around the next bend, the beautiful deep-blue sea appears. The seagulls cry in the air and on the long yellow sandy beach there are many umbrellas, sun loungers and beach chairs. "So, we've arrived!" calls train driver Egon as he brings the train to a stop at the nearby station. While the carriages are unloaded and reloaded with new cargo, Uncle Egon has the day off and can go to the sea with Paul and Kira.
First he buys them each a huge ice cream and then they explore the beach together. The waves form little white crowns of foam and lap evenly ashore. Kira and Paul giggle with excitement the whole time. Eventually they find a nice spot where they can build a sandcastle. Uncle Egon helps them with it and then settles into a beach chair to rest.
The day at the beach flies by and Paul and Kira are really sad when it's time to go home. As a farewell they take a keepsake photo of the sea. And as they board the train again, Paul calls out happily, "Goodbye, dear sea. We'll visit you again soon!"
