When college acceptance and rejection letters are received, the kids must consider what path they are going to follow. Summer is accepted into Brown University, whereas Seth is not. Not wanting to hold back Summer from such an grand opportunity, Seth lies by telling Summer and everyone that he got accepted into Brown, and begins to treat Summer badly. Ryan and Sadie try to decide what Ryan will do after high school when he is accepted into Berkeley. Marissa is also accepted into Berkely, but does not know if she wants to leave Newport. Meanwhile, Sandy tries to resolve the conflict between the unscrupulous, and mob-connected Henry Griffin and the hotshot Matt, where Mr. Griffin tries to bribe Sandy into not filing charges against him. Accepted to a college in France, a joyful Taylor holds a college acceptance bonfire party where Summer and Marissa try to fix their friendship problems, and Sadie decides not to move to Berkeley with Ryan after seeing that it will hold him back and she leaves for Portland, for good.
- Dennis Quaid as Professor Jack Hall, government climatologist and father of Sam Hall.
- Jake Gyllenhaal as Sam Hall, teenage son of Jack and Lucy Hall.
- Emmy Rossum as Laura Chapman, love interest and classmate of Sam Hall.
- Ian Holm as Professor Terry Rapson, colleague of Jack Hall stationed in Scotland.
- Christopher Britton as Vorsteen, associate of Jack.
- Arjay Smith as Brian Parks, best friend and classmate of Sam Hall.
- Dash Mihok as Jason Evans, friend and colleague of Jack.
- Jay O. Sanders as Frank Harris, friend and colleague of Jack.
- Sasha Roiz as Parker, an astronaut aboard the International Space Station.
- Sela Ward as Dr. Lucy Hall, physician/doctor, wife of Jack and mother of Sam.
- Austin Nichols as J.D., rival turned friend of Sam.
- Adrian Lester as Simon, colleague and friend of Terry Rapson.
- Tamlyn Tomita as Janet Tokada, Hurricane specialist for NASA, colleague of Jack Hall.
- Glenn Plummer as Luther, a homeless man in NYC.
- Perry King as President Blake, President of the United States.
- Kenneth Welsh as Raymond Becker, Vice President of the United States, later President of the United States.
- Amy Sloan as Elsa, a woman who remains in the library and survives with the group.
- Sheila McCarthy as Judith, the librarian.
- Nestor Serrano as Gomez, director of NOAA.