Inheriting $1,450 upon the death of his aunt, gullible sailor Tim Dunnovan (William Bendix) purchases a race horse, to the dismay of his best friend, Benny Linn (Groucho Marx), who assumes Tim has been swindled. Tim and Benny learn that the horse, Little Erin, has weak ankles and is a poor racer -- but his devoted groom reveals that he has a twin, Little Shamrock, in excellent condition. Discovering the whereabouts of Little Shamrock, Benny schemes to switch the horses, but complications arise.