An invisible and mysterious force field descends upon a small fictional town in the United States, trapping residents inside, cut off from the rest of civilization. The trapped townsfolk must discover the secrets and purpose of the "dome" and its origins, while coming to learn more than they ever knew about each other.

Season three finds each member of the Gallagher family seeking their individual version of the “American Dream.” Jimmy has moved into the Gallagher home, and Fiona is beginning to realize that a long-term relationship isn’t nearly as fulfilling as she expected… and her GED will only get her so far. Meanwhile, Frank reappears south of the border, with no idea how he got there.

Frank and Monica rekindle their self-destructive relationship after Frank's mother returns from prison to wreak havoc. Fiona is thrown for a loop when Steve returns from Rio with a surprise. Lip makes big changes in preparation for the arrival of his and Karen's baby. Karen and Sheila clash over Karen's new husband Jody, a good-hearted but weird guy she met in Sexaholics Anonymous. Ian tries to get into West Point. Debbie prepares for the onset of puberty. Kev and Veronica contemplate starting a family.

Frank goes missing and turns up in Toronto, tries without success to embrace sobriety, and bends the law to protect his disability checks. Fiona falls for Steve, a handsome player with a nice car, a fat wallet, and a big secret. Ian embraces his sexuality, much to the chagrin of his brainy brother Lip. Debbie goes too far at a child's birthday party. Monica shows up with an announcement about baby Liam.