In the small town of Waterloo, Iowa, Thomas and Alleta Sullivan spend the early years of their married life happily attending the christenings of the latest additions to their Irish-American, Catholic family: George Thomas in 1914, Francis "Frank" Henry in 1916, Joseph Eugene in 1918, Madison "Matt" Abel in 1919 and Albert Leo in 1922. As the boys grow, they are doted upon by their mother and sister Genevieve and given stern but loving guidance by their father, who is a railroad freight conductor. The day before Al's first communion, the youngster persuades Alleta to let him wear his new suit to attend confession. The suit is ruined, however, when the boys, who do everything together, brawl with some neighborhood kids over their new dog, Chiefie. Alleta is dismayed by their conduct, but Father Francis allows Al to take communion after the boys assure him that they bear no grudge against their rivals. Later, the boys get into more mischief when they find a broken-down boat and caulk its holes with mud. The vessel stays afloat until it reaches the middle of a lake, then begins to sink. Chiefie aids in rescuing Al, thereby assuring his place in the family. Upset by Al's near-drowning, Alleta makes her sons promise that they will not set foot in a boat again until they are adults. Later, the youngsters are complaining about fetching wood for Alleta's stove when Tom makes an offhand remark about building a wood box in the kitchen wall. Wanting to help Alleta and save themselves some work, the boys obtain lumber on credit and begin building the wood box. George becomes irritated by Frank's imperious manner, however, and leaves the project. The boys then break a water pipe and the kitchen is flooded. George rushes to help, and when Tom arrives and surveys the damage, he assumes that George, as the eldest, is responsible. Before George can explain their good intentions, Tom slaps him, and the boy runs off. Frank confesses all to Tom, and as the evening wears on, each family member invents an excuse to search for the runaway. George reappears the next morning, and after reconciling with his son, Tom declares that this should teach the Sullivans to stick together. Years later, in 1939, the boys are grown and only Al is still in high school. On the day that George wins a motorcycle race, Al meets Katherine Mary, an only child who lives with her father. Despite their youth, Al and Katherine Mary fall in love, and soon Al shows Gen the engagement ring that he has purchased. Believing that Al is too young for marriage, his brothers tease Katherine Mary when she comes for dinner and convince her that Al has many girl friends. After Katherine Mary leaves in tears and Al sinks into despair, the brothers realize the damage they have done, and with Gen, Tom and Alleta in tow, they apologize to Katherine Mary. Soon after, Katherine Mary and Al are married, and ten months later, are expecting a baby. Al is fired for taking the afternoon off to escort his wife to the doctor, but his brothers vow to help them out. Later, months after little Jimmy has been welcomed into the family, the family is lolling about on Sunday, 7 December 1941, when they hear a radio report about the attack on Pearl Harbor. The boys realize that one of their friends was on the Arizona and resolve to join the Navy to avenge him. Al decides that he cannot go with his brothers, due to his family responsibilities, but when Katherine Mary sees his despondent face, she tells him to accompany the others to the recruiting station. The brothers tell Commander Robinson that they want to serve on the same ship, but Robinson states that the Navy can make no such guarantees. The brothers leave, but later, after George receives his draft notice, he writes to the Navy Department and obtains official permission for the boys to serve together. Later, Tom, Alleta and Katherine Mary eagerly await letters from their loved ones, who are serving aboard the Juneau in the Pacific. The fighting in the Pacific grows more intense as a battle rages off the Solomon Islands, and one day, the Juneau is hit. Four of the brothers find each other, then realize that George is below in sick bay. They rush down to get him, and when George insists they leave him behind, Al replies, "We can't go swimming without you." Soon after, Robinson visits the Sullivan home and tells Katherine Mary, Tom, Alleta and Gen that all five of the brothers were killed in action. Stunned, Tom goes to work and salutes the water tower on which his sons used to stand and wave to him. Sometime later, Tom, Katherine Mary and Gen, who has joined the WAVES, watch with pride while Alleta christens a new destroyer, the U.S.S. The Sullivans . As Tom and Alleta watch the ship sail away, Alleta declares, "Tom, our boys are afloat again."