The film opens with Mickey aboard a tiny raft out in the middle of the ocean. As the waves rock the raft to and fro, Mickey suddenly sees land ahead.
With an improper sail, Mickey has no way of getting towards the island; thankfully the Mouse is smart fella. He cleverly tricks a swordfish into becoming a motor.
Once on dry land, Mickey finds an odd box awash on the beach. The unforgiving waves crash down on both Mickey and the box, revealing a wet piano. Of course this is a dream come true for our hero.
As Mickey inspects the piano, he is accompanied by three seals. Not one to displease his audience, the Mickey decides to play some tunes.
From here the toon turns into a typical song-and-dance Mickey Mouse cartoon. An interesting note is that The Castaway re-uses animation of the seals dancing from 1929's Wild Waves. As you can tell, the animation is identical in both films (with a scenery change of course).
This film also re-uses animation of a gorilla from 1929's Jungle Rhythm. Unlike the re-used seal animation, the gorilla animation in The Castaway is just half of the scene from Jungle Rhythm. The gorilla is attracted by Mickey's fantastic piano playing.
As the curious gorilla inspects the piano, he carelessly breaks the instrument much to the chagrin of Mickey. An annoyed Mickey picks up a boulder in anger, but later regrets his hasty decision.
As Mickey sheepishly discards the boulder, he unfortunately is thrown out of the frying pan and into the fire. It seems what he thought was just a pile of rocks and a bush are actually the body and mane of a ferocious lion!
A chase ensues with Mickey running for his life from the hungry lion. Once Mickey thinks he is safe atop a rock in the river, he again is dealt another blow when a crocodile rises from the depths. But luck is on our hero's side: the lion leaps for Mickey but instead lands inside the croc's mouth. Mickey is saved from certain death.