When Claudius chose Agrippina the Younger in order to consolidate the Julio-Claudian family, and picked her son, the future Emperor Nero, to fill the role of temporary older heir, Narcissus allied with Britannicus' circle in order to secure his future.
He was charged with overseeing the construction of a canal to drain Fucine Lake, but Agrippina, now Claudius's fourth wife, accused him of embezzling funds from the project, possibly as punishment for his support of Britannicus.
According to Tacitus, Narcissus hoped to bring down Agrippina by revealing her affair with the freedman Pallas, which would also have destroyed her son.
Suetonius and Dio report that, after reconciling with Britannicus, Claudius, not Narcissus, openly planned to bring Agrippina down.
In any case, Agrippina was suspicious of Narcissus and had him sent away to Campania, ostensibly to take advantage of the warm baths there to relieve his gout.