Short answers, pulled from the story.
Nerva became emperor in 96 CE after a period of political instability. He chose Trajan, a popular military leader, as his adopted heir to reconcile tensions between the army and the senate.
The first five rulers from 96 CE to 180 are known as the Five Good Emperors. These leaders governed with moderate policies unlike their more tyrannical successors according to Niccolò Machiavelli and Edward Gibbon.
Hadrian died in 138 after a long reign dedicated to cultural unification. He shifted focus from expansion to defensive consolidation by building fortifications along the northern frontiers to secure the empire.
Marcus wrote philosophical reflections that survive today as Meditations revealing his commitment to Stoic philosophy while governing. His decision to name his biological son Commodus as heir instead of adopting another capable leader broke the tradition of selecting successors based on merit rather than bloodline.
Commodus ruled from 177 CE until his murder in 192. In 192, conspirators assassinated him to end his reign which marked the end of the Nerva Antonine dynasty.