For the many Beardsley kids, constantly moving to new places is a fact of life, especially when their dad Frank (DENNIS QUAID) keeps moving up the chain of command in the U.S. Coast Guard and may soon replace his superior (RIP TORN) as the commandant of the entire organization.
Even so, it isn't easy for his eight kids -- Ethan, 4 (TY PANITZ); 6-year-old twins Otter (BRIDGER PALMER) and Ely (BRECKEN PALMER); Kelly, 8 (HALEY RAMM); Harry, 10 (DEAN COLLINS); Michael, 12 (TYLER PATRICK JONES); Christina, 16 (KATIJA PEVEC); and William, 17 (SEAN FARIS) -- especially since their mother has been dead for a number of years. House manager Mrs. Munion (LINDA HUNT) is there to help around each new house, but with Frank's tightly disciplined family unit, things run rather smoothly.
That's in sharp contrast to widow Helen North (RENE RUSSO), Frank's high school girlfriend who's now an artsy, clothing designer whose manager, Max (JERRY O'CONNELL), is trying to land her a lucrative department store job. Helen thrives on disarray and chaos, two conditions in which her kids -- Aldo, 4 (NICHOLAS ROGET-KING); 8-year-old twins Marisa (JESSICA HABIB) and Bina (JENNIFER HABIB); Lau, 9 (ANDREW VO); Joni, 10 (MIRANDA COSGROVE); Jimi, 11 (LIL' JJ); Mick, 12 (SLADE PEARCE); Naoko, 14 (MIKI ISKIKAWA); Dylan, 16 (DRAKE BELL); and Phoebe, 17 (DANIELLE PANABAKER) -- thrive.
Their paths meet again when Frank is reassigned to run the Coast Guard Academy in New London, Connecticut alongside old friend Darrell (DAVID KOECHNER). It's there that Frank runs into Helen at their high school reunion. Sparks and feelings are immediately rekindled and the two get married on a spur of the moment whim -- much to the shock of their kids who are then forced to move in together in a large lighthouse home along the coast.
With the siblings being unaccustomed to their new step-parent's different style of parenting as well as the behavior and attitudes of their new stepbrothers and sisters, the eighteen kids end up clashing. Realizing they were better off before this new scenario, they decide to put aside their differences and conspire to do whatever it takes to drive a wedge between their parents in hopes that they'll eventually split up.