You can begin this Stranger mission sometime during the middle of Chapter 3, so keep checking the eastern side of Rhodes for the familiar white circle. When he's there, you should find a man by the fence in front of the church. After apologizing profusely, you find out that some men stole his wagon full of medical supplies, so you decide to go and get it back for him. As you're about to leave, a townsfolk will chime in and say that they were taking the wagon to some place called Macomb's End.
Although the game will give you a line to go there, it is located on the little peninsula where the "Y" in Lemoyne is on the map. Once you arrive at the destination, the enemies there won't be too friendly and things will devolve straight into a shootout. The first thing you should do is find some cover, like the trees nearby, as they should be big enough to avoid being shot. There will be about six enemies here, so kill them all, then locate the wagon you need behind the second building on the right. Board the wagon and drive it back to the doctor in Rhodes to finish the mission, noting that there will be a few more enemies on horseback on the ride back. You will receive the Special Health Cure Pamphlet as a reward, so make sure you read it to learn the recipe.