In the town of Maryville, residents are gathered in the canine cemetary to pay tribute to Rex, a German shepherd who rescued Captain William Joyce while in battle during World War I. Joyce recalls his story: His hopes of becoming an architect are dashed when his friend, John Fenton, registers Joyce's drawings in his own name at school. Not long after, Joyce is drafted, and he regrets leaving his mother and his faithful companion, Rex. Rex is also drafted by the Red Cross to be trained as an ambulance helper. When Joyce becomes terrified during an attack in the trenches one day, he escapes and cowers in a deserted, war-torn village. Rex is in an ambulance nearby that has a flat tire and, sensing Joyce's turmoil, trots off to find his old friend. Joyce is overjoyed to see Rex, and the dog's faithfulness encourages Joyce to return to his unit, where he has not been missed. After this, Rex never leaves Joyce's side and becomes useful to the squad. The forces continue to fight in France, and when Joyce is chosen by his commander to cross enemy lines to discover where wires connected to American detonation points have been cut, he is shot. Rex savagely attacks an enemy soldier who is about to finish the job and thereby saves Joyce's life. Rex pulls the wire and enables Joyce to reconnect them. After the massive explosion, the American troops emerge and commence their battle. Joyce recovers in an army hospital with Rex by his side, and both receive medals of valor. Upon full recovery, Joyce is assigned to air service. Flying in a biplane with Rex in the front seat, Joyce shoots down an enemy zeppelin. Joyce accomplishes many other heroic feats and his letters home reflect the fact that while once he was a timid boy, he is now a grown man and ready to give John the thrashing he deserves for stealing his drawings. When the armistice agreement is signed in Nov 1918, Joyce and Rex are welcomed home by a parade, his girl friend and parents. Joyce sees the library built with his design, and he knocks out John, who confesses to his treachery as Rex proudly looks on. The statue of Rex is unveiled.
The viewed print was incomplete and had no credits. In 1932, the picture was approved for exhibition in New York by the New York State Censor Board. At that time, the title was listed as The War Dog. The 1932 version, for which no reviews have been located, appears to have incorporated large portions of the 1925 silent film His Master's Voice, with some sound sequences added. The above cast and production credits were taken from copyright records and a Variety review for the silent film. Although the plot remained essentially the same, the character names in the 1932 film were different. Also, Thunder is billed as the canine star of His Master's Voice, whereas Rex is credited with the title role in The War Dog; it has not been determined whether these dogs are the same.