Bao Bao loves his friend Cassandra, but winter is arriving in Asia. Cassandra wants to leave Asia, but Bao Bao won't say "Goodbye" to her.
The animals talk to Lucy if she's ready for a vacation. Lucy is not actually ready yet; her best friend at school is leaving the whole summer to visit his cousins in the USA. Suddenly, a hare named "Horace" has appeared next to Lucy. This reminds him how Bao Bao won't let Cassandra leave Asia.
Bao Bao and Cassandra were best friends in Asia. They spent time during the summer. They played with bamboos, drank tea, watched clouds, and exercised all summer. As summer is coming to an end, Cassandra must leave Asia when autumn is coming. But Bao Bao is very upset that Cassandra is about to leave.
Bao Bao began asking Cassandra that tomorrow the two would play together. The next day, Bao Bao met Gertie near the grass. Bao Bao doesn't have a good time with Cassandra preparing to leave for autumn. Gertie began asking Bao Bao if he cleaned up his home, but Bao Bao refuses to talk to her. Bao Bao finally understood that it is important to say "Goodbye" to Cassandra.
Later, Bao Bao tried to say "Goodbye" to Cassandra as soon as possible. Cassandra began asking Bao Bao to play with bamboos, but Bao Bao wants her to leave. Cassandra won't leave at first. Bao Bao is very cranky that Cassandra won't leave. That is because Cassandra can only leave during the winter. Gertie appeared right next to Cassandra and discussed with each other about Bao Bao being wrong.
Later on, Horace appeared right next to Gertie to say it was she who told Bao Bao to stop playing with Cassandra. Bao Bao had a conversation with Horace why Cassandra won't leave the mountains today. It is explained that Cassandra will freeze during the winter in the mountains. Horace began asking for advice from Confuse-us about why it is important for someone to leave. He began replying that anyone may return after leaving.
Horace returned to Gertie eating flowers before winter. Bao Bao asked her to eat them before winter. Finally, Cassandra has arrived for her final moments with Bao Bao. Later on, a flock of cranes started leaving during the winter. Cassandra followed the flock of cranes and left the mountains. The animals said "Goodbye" to Cassandra. Cassandra asked the animals that she and her species will return next spring.
Lucy has understood why it is important for someone to leave. Lucy explained that her best friend has promised to send lots of postcards after vacation. The animals saw the postcards delivered by her best friend.