I think this is Badian's theory, although it might have predated him--it's one of the more prevalent theories behind the conspiracy, that Antony was in on it, since a lot of what Antony did before and immediately following don't necessarily make much sense. Antony certainly knew about the plot beforehand. Or rather, he knew something at sometime. Antony claimed that he had been tipped off by Casca only the night before that there was a plot to murder Caesar, but it seems unlikely that this was the first that Antony heard about it. That something was up was pretty generally suspected--Cicero assumed that there was a plot, for example, and tried to get people he suspected were in on it to give him information about it. There had been an earlier plot, that Antony had been in on, and had exposed to Caesar, and it wasn't exactly a leap of imagination to suppose that there would be other attempts--rumor spread fast among the senatorial class. When Antony exposed the earlier conspiracy it's unlikely that he did so out of any loyalty to Caesar, though it is possible--Antony had recently been subjected to harsh punishment and was demoted to a private citizen by Caesar following his total mismanagement of Italy while Caesar was away in Spain. Antony's bumbling attempts to deal with Dolabella's legislation earned him the hatred of many of Caesar's supporters and totally ruined his standing with Caesar--by exposing the plot he suddenly proved to Caesar that he was loyal, and Caesar started awarding him honors again, such as the consulship in 44, and trusting him. Whether Antony really had done it out of loyalty it was still extremely self-serving. But the circumstances of the plot in March were very different--Antony was back in Caesar's good graces, and Caesar was, if anything, now a limiting factor, preventing him from rising any further. Antony's excuses are a bit too convenient--he knew about the plot and was rushing to warn Caesar, but he just so happened to be delayed by a group of senators who shooed him away. One might expect that had Antony really been determined to warn Caesar he would've paid them no heed--what were they going to do, beat him up if he didn't stop to chat? It's a rather flimsy excuse, but it is an excuse--like much with Badian's theory, it's hard to prove, though things do look rather suspicious. Another issue is that Casca is said to have warned Antony and was supposedly in a panicked and terrified state--but Casca appears to have gotten over his anxiety rather quickly, as he was the first to approach Caesar and struck the first blow. Again, this seems rather like an excuse from Antony, an attempt to explain why he knew that Caesar was going to be killed, hence his haste (or lack thereof, depending on whether we believe Antony's story) to warn Caesar. Antony was also by far the greatest beneficiary of Caesar's death, or at least seemed so. Nobody, not even Caesar's wife, was aware that Caesar had changed his will after Antony's governorship of Italy, largely writing out the man who had proven himself an incapable administrator and a doofus when unsupervised. Antony expected, like everyone else, that Caesar's will would adopt him, not Octavian, who was a nobody from an obscure family. In the days after Caesar's death Antony found himself in an incredibly strong position, as sole consul and leader of all the Caesarians (and their troops, whom Lepidus marched into the city on the 16th with the intent to storm the Capitol, where the conspirators had barricaded themselves). Antony managed the first few days quite adeptly, preventing Lepidus from using violence and bullying the conspirators into accepting a settlement--he was now the leader of the Caesarians and the benefactor of the conspirators, and until Caesar's will was read on the 19th his position seemed bulletproof. Seems rather convenient--it's also been argued that the settlement with the conspirators might have been set up in advance, since it was a bit too neat and orderly, but I think that's rather hard to demonstrate (Antony had Caesarian troops breathing down their necks, after all)

Mind you, all this is very very circumstantial evidence, but it is a theory that exists and has a fair amount of traction.

In spite of his closeness to Caesar, he knew that the assassination was being planned; he had treacherously kept the information to himself.

I have a question of my own. Does Everitt provide any sort of evidence supporting his argument that Antony was knowledgeable about the impending assassination? I'm interested to know because - like you - I have never come across anything suggesting this.

Can anyone recommend a history that focuses on the era from Caesar's assassination until Augustus's final triumph?