The general situation of the play is established almost from the very first scene. It is in Clov's slow, almost ritualistic actions, though, that the scene is revealed to us. He looks out each window and laughs, though we are not yet sure at what. He then comes over to the center of the room and, in a literal unveiling, pulls back sheets to reveal Hamm and Nagg and Nell's trash bins. After this point, very little changes. We quickly learn that the situation is set, and that the character's fates were determined long ago. As with the ending, the tragedy has already taken place. The characters are simply drawing it out.
The dream stages are when various characters reach back to the past in an attempt to free themselves from the present. In almost all cases, however, these moments of recollection either fail to take hold or they simply cast their characters back into their present torment. For example, Nagg and Nell remember falling off their tandem together and losing their shanks (which brings them to the present, where they are legless and stored in trash bins). They then remember a time that Nagg told a joke on Lake Como, though Nell cannot appreciate it. She becomes wrapped up in the details of the scene and dies shortly after.
Clov often thinks longingly of yesterday or of times when he was young, though pretty much his entire life has consisted of serving his master Hamm. Hamm, for his part, looks back on the time when he first obtained Clov, but finds no great revelation in the memory. Instead, it brings him right back to his current situation, and he is left wondering when things are going to end. Still, even though these moments of recollection don't change anything, they are among the happiest in the play.
The dialogue between Hamm and Clov revolves around the problem of ending, and whether or not they will be able to achieve the task. They often become frustrated with one another. They imagine that their situation cannot actually get any worse, and are absolutely frank about their own inability to do anything about it. They sometimes try to crack jokes with one another or to be spontaneous, but their exchanges have a repetitive quality that suggests that they've had these conversations many times before and they feel very stale to them.
Though the characters often imagine that they cannot be worse off, Hamm threatens Clov with a vision that is even worse than his current state, a threat that Nagg uses against Hamm later on. Hamm predicts that Clov will at one point be nothing but a speck in the dark with no one to care for him or pity him. Nagg, for his part, actively wishes that Hamm were alone in the dark, crying for his father as if his father were Hamm's only hope.
Both of these visions hit on the theme of complete isolation and total abandonment: the individual is alone and the world has disappeared into darkness. This is not a vision of death but of another form of life, before the End, that would be even worse than their present, sorry existences.
On the one hand, the ultimate tragedy never comes. There is no final destruction or death (except for Nell). On the other hand, the ultimate tragedy has already taken place before the play began.
The end of the play, when it is unclear whether or not Clov will actually leave Hamm, brings us back to the beginning. It draws attention to how little Clov's choice matters and to the possibility of the following day looking a lot like the one we just witnessed. The horror of the character's situation is especially apparent at this moment because nothing has changed. In contrast to what they are going through day by day, real death might just be a welcome release.