In a discussion of irony, characterization and plot, I can give only the briefest remarks since Answers on eNotes are limited to space and since our policy is to aid understanding of assignments but not to do student assignments.
Irony is rife throughout the novel. Both verbal irony (what is said has another figurative meaning) and situational irony (the situation is contrary to what is expected) occur in abundance. It is not wearying that there is so much irony, though, because it adds humor and helps characterize the first-person narrator: a first person narrator can directly describe themselves in only a limited and most often clumsy way, so techniques like irony develop characterization in an indirect manner. Ironically, Amis also uses irony to develop the setting (time and place), which is an unusual technique for establishing setting. An example of situational irony developing setting is:
Features include ... a removable troupee of rust on the hood, and adhesive key-scratches all over the paintwork. An English strategy: envy preemption.
The situation is that the narrator is admiring Aspery's new car moored out at his cottage. The irony is that the new car is decorated to look like an old heap of a rusted, scratched up wreck. This situational irony helps place the time as some future time after some hinted at change in the world makes life and prosperity more dangerous.
Since the first-person narrator has center stage for the exposition, the plot set-up comes directly from his comments, especially those relating to what he is doing or about to do. An instance of this comes at the end of the exposition where he segues from the frame to the interior story, the one he is in London to write. His comments tell not only, for example, that he has bartered with another writer for a writer's retreat in London, complete with A-to-B car, but that we are going to be reading along as he writes his "snappy little thriller":
I am ready to write. ... If London is the fly, where do I fit in? ... I'm the fly. ... Let's start with the bad guy. Yeah. Keith. Let's start with the murderer.
Once the narrator begins writing "Chapter 1," he employs a mode of characterization that is direct characterization in combination with indirect characterization. For example, he describes Keith directly in some points: e.g., he was a bad guy and he was "multiracial in outlook." Additionally, though, Keith is described indirectly through his relationships and encounters: "He had saving graces. ...strange-hued women had sweetened him somewhat." Most interestingly, Keith is characterized by juxtaposed contrast to what he is not: "Within those eyes [of "these worst guys"] a tiny unsmiling universe."