On a sunny afternoon Paul and his friends, the cat Kira and the dog Hugo, meet at the playground and play a fun guessing game. "I spy with my little eye something that is ... blue!", calls Paul. Kira and Hugo look around and start guessing cheerfully: "The sky?" – "The slide?" – "The car over there?" – "The sea!!!", a voice calls from above their heads. "I have seen it often from the air." Paul and his friends look up in surprise. It is the parachuting frog, who is making one of his brave jumps again and lands with a flourish in the sandpit. Then he folds up his parachute and hops away in a hurry.

Kira and Hugo keep guessing for a while what Paul might have meant, but they just cannot find the answer. "The last carriage of Uncle Egon's railway is blue!", Paul finally calls. "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. "What if we ask your uncle to take us to the sea sometime?", suggests Hugo. Paul and Kira are instantly excited by the idea. Paul's uncle is very kind and is happy to take them on a day trip to the sea – but first they must still ask their parents for permission. They are due to set off tomorrow.

Paul can hardly sleep for excitement because he cannot wait to finally see the sea! Unfortunately, overnight Hugo has developed a sore throat and a fever and cannot accompany Paul and Kira. "Have fun at the sea!", calls Paul's mum. "We'll collect you 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 journey – past brown fields and green meadows where plump cows graze. And then, after a while around the next bend, the beautiful, deep-blue sea appears. The seagulls cry overhead and along the long yellow sandy beach there are many parasols, sun loungers and beach chairs. "Right, we're here!", calls engine driver Egon as he brings the train to a halt at the nearby station. While the carriages are unloaded and reloaded with new cargo, Uncle Egon is off duty and can go to the sea with Paul and Kira.

First he buys a huge ice cream for the two of them and then they explore the beach together. The waves form little white foam crests and lap evenly ashore. Kira and Paul giggle with excitement the whole time. Eventually they find a lovely spot where they can build a sandcastle. Uncle Egon helps them with it and then makes himself comfortable in a beach chair to rest.

So the day at the beach passes in a flash and Paul and Kira are truly sad when it is time to go home. As a farewell they take a keepsake photo of the sea. And as they board the train again, Paul calls happily: "Goodbye, dear sea. We'll visit you again soon!"