Let's just say right up front that this play doesn't exactly fit a classic plot analysis. There is a lot of waiting in the play, with action stopping and starting. Because Ros and Guil are so passive, the plot seems to happen almost in the background in the play. In many ways, their inertia and passivity takes the foreground. Yet, many elements of the play – the relationship between Ros and Guil, the role of chance, the interplay between art and reality – all come out in these early moments of Act One. It's also worth noting that, once the Player arrives, all the major characters have already been introduced.
This is a minor scene, but it's the one that sets off much of the action of the rest of the play. It might create more of a conflict for Ros and Guil if they were more tuned in to the fact that Claudius is Hamlet's enemy. By agreeing to figure out what is wrong with Hamlet for Claudius, Ros and Guil get themselves entangled in the events of Hamlet – events that will ultimately lead to their deaths.
Things get complicated in two different ways. First, as the Tragedians perform their dress rehearsal for Ros and Guil, this questionable relationship between art and reality comes more to the fore. It's apparent when Guil begins arguing with the Player about whether or not someone can act out death, and it's apparent when it becomes clear (to the audience anyway) that the play the Tragedians will put on prophesies everything that will happen later on in reality (reality here being the play that the audience is watching).
In terms of action, this is definitely the climax. Tension between Guil and the Player has been cooking since Act One, when Guil slaps the Player, and it is here that he simultaneously tries to make his point (there is a difference between reality and art) and takes out his aggression on his adversary. Note that the climax involves Ros, Guil, and the Tragedians. Hamlet is not even on stage – this makes it very clear where the emphasis of Stoppard's play lies.
The play might be seen to suffer from a lack of suspense, since it sticks to the events as they unfold in Hamlet. Guil himself comments on the lack of suspense in the initial scene while they flip the coin. Yet the suspense is most clear when he and Ros act out the scenario and find out, first, that they are betraying their friend by taking him to England (where he will be beheaded) and, second, that they have been betrayed by their friend on the way to England (where they will be killed). The audience clearly knows what's coming, but it makes it that much more painful when Ros and Guil, who are oblivious, find out the nature of their fate.
One could argue that all of Act Three is just the denouement. This is not to say that there is no suspense in the act, but the fact that Ros and Guil are on a boat underlies the fact that the course of the play is now determined and their fates are sealed. Thematically, however, there is a great deal of suspense when Guil argues with the Player and then stabs him (in an attempt to show the difference between acted and real death). When the Player stands up after his performance, Ros and Guil are more or less entirely resolved to their fate.
Stoppard's play shares its conclusion with Shakespeare's Hamlet, but makes a significant commentary on it. At this point, Ros and Guil are dead – or at least disappeared – and so are Hamlet, Claudius, Gertrude, Polonius, and Laertes. According to the Player's criterion – everyone who is marked for death dies – this is the conclusion of the play. The scene differs from Hamlet because Horatio's speech, which promises a true recounting of what has taken place, is drowned out by darkness and music.