On any given day, a classroom visitor should be able to walk over to a group of students and ask them- What are you working on? Why are you working on this? Students should be able to answer by explaining a question they are trying to figure out or a problem they are trying to solve, and not just say because the teacher told us to do this.
A storyline is a coherent sequence of lessons, in which each step is driven by students' questions that arise from their interactions with phenomena. A student's goal should always be to explain a phenomenon or solve a problem. At each step, students make progress on the classroom's questions through science and engineering practices, to figure out a piece of a science idea. Each piece they figure out adds to the developing explanation, model, or designed solution. Each step may also generate questions that lead to the next step in the storyline. Together, what students figure out helps explain the unit's phenomena or solve the problems they have identified. A storyline provides a coherent path toward building disciplinary core idea and crosscutting concepts, piece by piece, anchored in students' own questions.
What makes a storyline different from just a sequence of lessons?
Often the importance of a particular problem or idea is clear to the teacher, but not to the students. For example, the teacher knows how learning about the cell will help with important biological questions; but for students, they are learning about cells because that's the title of the current chapter in the textbook. The teacher may know how a particular chemistry experiment will teach students something about conservation of matter; but to the students, they are doing the experiment because they are following the directions. In a storyline, students should be involved in co-constructing the question we are working on, and should see the activity as helping make progress on that question. In a storyline, the coherence is from the students' perspective, not just the teacher's.