Shep the Sentinel.

When May receives a letter from another girl telling her that Jack, her lover, is untrue, she dismisses him. Shep, Jack's beautiful collie, however, is suspicious of Jim Hallet, who, some way, he feels is responsible for his master's unhappiness. He trails Jim and steals another letter, written by Nell Borden. Jim's accomplice, which betrays the plot. This Shep brings to Jack, who then is able to convince May that trickery and Jim's jealousy are behind their estrangement. Meanwhile, Jim has schemed to enter May's house by night and commit a robbery, putting the suspicion upon Jack. But Shep, whom Jack has given to May, hears the thief. The noise of their struggle wakes May, who summons Jack and the police.