Mickey and Pluto are decorating the tree but some unexpected guests (Chip and Dale) have made their home in the tree and are dazzled by all the lights and decorations. Pluto sniffs them out and ends up destroying everything trying to catch them.
This classic Mickey Mouse is a must see every year. At this point in Mickey's career, most of his cartoons starred him but the spotlight was always stolen by his co-stars to the point where he would get very little screen time. In this case it's Pluto, whose career was at the height of it popularity, and Chip n Dale, who had just reached the height of their own popularity.
While the animation was really good on the chipmunks and Pluto, I am not a fan of Mickey's animation in this era of his career. The artists thought to change him up a bit. They made his ears not as round and his nose a bit longer. He never wore his trademark red shorts anymore and he moved like he had no neck. This cartoon is a good example of how even the best animators can make Mickey look "wrong".