Sad Sack is the story of Meredith Bixby (Jerry Lewis), a private in the US army, who proves himself a serious liability in basic training. After an incident involving the loss of a tank, the psychologist Major Shelton (Phyllis Kirk) is given the task of transforming him into a good soldier with the help of Corporal Dolan (David Wayne) and Private Wensalawsky (Joe Mantell). Unfortunately for Bixby these two hold a grudge against him and set him up several times, resulting in even more calamities. All of them are shipped out to Morocco and, while on leave there, Bixby falls in love with the dancing girl Zita (Paula Hill) but Dolan, in a bid to protect Bixby, tells him lies about Zita. Despondent Bixby joins the Foreign Legion and becomes kidnapped by a band of Arabians. This results in a rescue mission by Dolan and Wensalawsky, with the assistence of Zita.