Crosby is Jack's pet dog sharing impoverished countryside life with Jack and his mother. He witnesses Jack trading their cow for magic beans and reacts with astonishment when the beans sprout overnight into an enormous beanstalk. His surprise deepens upon seeing a mouse named Laura descend wearing a dress and hennin. Crosby observes Laura convincing Jack to climb the beanstalk and joins their ascent.

Reaching the castle courtyard, Crosby accompanies Jack and Laura to encounter the enchanted Princess Margaret. He remains by Jack's side during their explorations. Crosby avoids detection when Madame Hecuba attempts to drug Jack with enchanted soup and flees alongside Jack at giant Tulip's arrival. He participates in meetings with enchanted mice Nigel, Tammy, and George, and hears a talking harp reveal the castle's curse. Crosby witnesses Jack acquiring treasures, including a golden hen, and joins their escape down the beanstalk.

After returning home wealthy, Crosby persuades Jack to climb again to rescue Margaret from her forced marriage to Tulip. He accompanies the second ascent, assists in disrupting the wedding ceremony, and participates in the ensuing chase after Jack breaks Margaret's spell with a kiss. Fleeing the castle alongside Jack following Hecuba's defeat and Tulip's turn against his mother, Crosby descends the beanstalk while pursued by Tulip. He stands by as Jack cuts down the beanstalk, causing Tulip's fatal fall, and observes Jack reflecting on their friends in the sky castle.