The treecat Swift Striker was attracted to him because of his strong mind-glow, a trait he inherited from his grandmother, Granny MacChait. The adoption came about when he was out fishing and slipped in the river, seriously lacerating his head and possibly fracturing his skull and a leg. He would have drowned, but Swift Striker kept his head above water with his carry-net and fashioned crude bandages for his head wounds from MacDallan's shirt sleeves. Because of his mental sensitivity, he was among the few humans who very early understood how intelligent treecats really were.
He, with the urging of Fisher and an apparently abandoned treecat, True Stalker, uncovered the fact that Dr. Muriel Ubel had caused an environmental disaster and to cover it up, murdered True Stalker's human, Arvin Erhardt, by causing his aircar to crash. MacDallan never explained the truth of his investigation, nor the actual truth of the deliberate sacrifice of the stray. He chose to remain silent to protect the treecat's secret: that they were actually much more intelligent than they let humanity know at the time. (HHA2.1: TS)
Some time after the BioNeering scandal and the media frenzy surrounding it, MacDallan met with Stephanie Harrington and her parents to discuss their knowledge of the treecats' true intelligence. He also came to Stephanie's aid when she caught Tennessee Bolgeo trying to kidnap treecats. (SK1)