Mark and his father have a verbal showdown after which his father finally gives him a year to prove himself and his horse, or he must sell the horse. During this time, Mark and Jaguar learn about each other sometimes humorously and sometimes painfully.
Mark and his friends become involved in a mystery when there is a forest fire that threatens to consume all of the Long Island area where they live. This problem is compounded when the stable where they board their horses is threatened with closure because horses are being blamed for polluting the nearby river. When these problems appear to be solved, Mark has an even greater one when he accidentally discovers Jaguar is blind. There is no money to pay for an operation.
A man who had befriended Mark and Jane and given Mark animal care books, comes forward and admits he is a retired veterinary surgeon. He offers to operate and the only payment he requires is that Mark "pass it on" when he becomes a vet. The operation is a success and Mark and Jaguar go back to barrel racing.
On the day they win their first "first,' Mark's father tells his son he has shown wisdom and maturity in handling his problems and he may keep Jaguar and continue on with his racing.