Plot is composed of causally related events that escalate toward a climax.
Events are causally related when one action causes another—because A happened, B happens. Take, for instance, the first 4 chapters from Dean Koontz's Sole Survivor. The protagonist, Carpenter, comes to a beach. This is his action. He comes to the beach before heading to the cemetery where he will visit his wife's grave. What happens as a result of going to the beach? Carpenter discovers that someone is watching him, or at least seems to be watching him. If Carpenter has not gone to the beach, he would have not become aware of the surveillance, and the subsequent action in the cemetery would not have been set up or foreshadowed. One action causes (sets the stage for, sets up) other actions. Carpenter, despite the possibility of being watched, (he doesn't care about being watched, and this expresses his depression) proceeds to the cemetery anyway, and this is where a crisis occurs. Because he went to the cemetery, he discovers that nothing about the death of his wife is as it seems, and is plunged into the conflict. If he had not gone to the cemetery, he would not have met the person who claims to be a survivor of the crash, and he would not have found the motivation to discover the truth behind the crash. The protagonist has acted, and because he did, something happened that changed his situation: Carpenter is transformed from apathy and suicidal depression to the rage motivated desire to find out the truth about his wife's death. But events cannot, in a good plot, merely cause one another--events must cause other events that occur on a higher level of escalation.
Events escalate when one action causes another action that is greater in magnitude, impact, intensity, or reaches a new level. Soul Survivor shows escalation at work in the first 4 chapters: Carpenter is, at first, merely watched, which he becomes aware of at the beach. This is the beginning of change of his situation. This situation escalates, evolves, when he comes at the cemetery, where he encounter the survivor—this encounter is the a surprise, one that ups the ante, increases the impact and intensity of events for Carpenter, and for us as readers—he had been mourning the loss of his wife, now it appears that people not only survived the crash, but that some sinister force wants to keep the truth about the crash secret.