Charlie plays a private in the army at a training camp at the time of the First World War. Although his drilling squad is not very capable, Charlie outshines the other trainees in incompetence.  After being on his feet for a long time, he gets into his bunk and goes to sleep.  He dreams of what he will do on the battlefield overseas.

Arriving at the trenches equipped with an enormous amount of gear, he volunteers to go into enemy-occupied territory.  He covers himself with a tree trunk in order to move about freely and has some hazardous but funny encounters with some German soldiers while in his disguise.

When he comes upon a shattered house, he finds a French girl still living there.  The girl hides Charlie when German soldiers arrive.  He gets away, but the girl is taken into custody by the Germans for harboring an American soldier.  Charlie, however, stumbles upon the headquarters of the German command where the girl is being held. He knocks out a German officer and puts on his uniform.  The Kaiser arrives and accepts Charlie as the legitimate article.  Charlie rescues his company sergeant, who has been captured, and sends him with the girl to the Allied lines.

Charlie then chauffeurs for the Kaiser and his companions, the Crown Prince and Von Hindenberg.  He drives the car to the Allied lines and turns the three leaders over to the Allied command.  Charlie is a hero but his dream comes to an end when two soldiers awaken him and tell him that it is time for him to get on with his training.

One of the greatest of the Chaplin films, Shoulder Arms was released only a few weeks before the Armistice.  It was originally planned for five reels and its unused footage has been preserved by Chaplin.

What was said about Shoulder Arms:

Chicago Herald
Shoulder Arms is very, very funny.  Mr. Chaplin, with his sad seriousness, makes a delicious doughboy and gets into situations amazing even for him.  Laughter follows on his every movement and loud applause when he bags the person who is seeking safety in Holland (i.e., the Kaiser).  And when he hates to get up in the morning―oh, me, we feel almost the sympathetic tear.  It's a bravely jolly little picture, excellently done, and a concentration of brilliancies, in a comedy way, like the light-shooting facets of a diamond.

The New York Times
"The fool's funny," was the chuckling observation of one of those who saw Charlie Chaplin's new film, Shoulder Arms, at the Strand yesterday and, apparently, that's the way everybody felt.  There have been learned discussions as to whether Chaplin's comedy is low or high, artistic or crude, but no one can deny that when he impersonates a screen fool he is funny.  Most of those who go to find fault with him remain to laugh.  They may still find fault, but they keep on laughing. In Shoulder Arms Chaplin is as funny as ever.  He is even more enjoyable than one is likely to anticipate because he has abandoned some of the tricks of former comedians and introduces new properties into his horseplay.  His limber little stick, for instance, which had begun to lose its comic character through overwork does not appear.  Instead Chaplin, camouflaged as a tree trunk, plays destructively with one of the tree's branches.  The baggy, black trousers are also gone, giving place to a uniform and such equipment as a soldier never dreamed of.