Arthur, the frail younger son of affluent English widow Mrs. Milligan and biological brother to the protagonist, lives constrained by paralysis, reliant on a wheelchair and the constant attention of his maid Vivian. His fragile health fuels a solitary existence, coloring his early interactions with melancholy and seclusion.

During a journey aboard the riverboat *Swan*, he forges a connection with the protagonist, whose playful demonstrations of animal tricks and music briefly pierce Arthur’s isolation, offering companionship. Their bond deepens when a buried family secret surfaces: the protagonist is Mrs. Milligan’s infant, stolen years prior, making the pair siblings.

Arthur’s condition propels his mother’s desperate European pursuit of experimental cures. Years after the central events, he achieves a dramatic physical recovery, learning to walk as the family’s fractured history mends. This healing parallels the resolution of their protracted search for the lost heir and the unraveling of criminal schemes within their orbit.

Inheritance conflicts rooted in English succession law, which once threatened Arthur’s position as sole heir, dissolve once the protagonist’s lineage is proven, securing Arthur’s place within the restored family.