4 ways your protagonist can learn the truth
This is perhaps the simplest causal structure outlined here, whereby the MC discovers a series of indicators that eventually lead him to the truth. One discovery leads him to a place where he discovers another piece of knowledge or understanding which leads him to another place where he discovers something else etc. etc.
The pieces of knowledge could be a clues, insights into a personal trauma, understandings about a relationship he didn't have before or a mixture of all the above – anything that is appropriate to your story. The key is that each discovery drives the character onto to the next – hence 'the chain'.
This is where a series of possibilities are known or suspected by the MC and the story is about him discovering which one of these is correct. In the simplest sense this could be a detective trying to discover the murderer from a fixed set of suspects – the classic Agatha Christie-esque situation – but it could equally apply to a whole heap of other stories. The MC then (either knowingly or unknowingly) eliminates all the possibilities until alighting on the correct one.
You can have all sorts of fun with this set-up – using dramatic irony where the audience/reader knows the truth while the MC doesn't, or keep the reader as ignorant as the MC. Exceptionally clever writers can convince the MC and the reader that one possibility is a dead certainty, only to discover it's false and a least suspected option is actually the case.
This is where the truth seems simple to the protagonist but the situation is far more complex than he imagines. A classic example of this is where an event occurs, which could be quite innocuous, but there turns out to be a massive conspiracy behind it. With 'the convergence' the solution is simple and manageable, but with 'the divergence' it is anything but. Perhaps the MC can do something about it, perhaps the reality is so large (or cosmic) that he can do nothing at all. One way to make a distinction between 'the convergence' and 'the divergence' is that in the former the problem gets smaller and more is known, but in the latter, it gets bigger.
Probably the simplest and cheapest device of the lot – where the knowledge comes right out of the blue for the character. This can be a twist (re. Sixth Sense) or reveal (Darth and Lukes' relationship) – and can provide dramatic interest or insight, but used alone will not provide narrative drive unless it leads on to the more causal structures outlined above.
Obviously your not compelled to use only one of these devices, you can use them all – and it's worth noting that these aren't the only ways to provide narrative drive. Mash it up and use of many of them as you can, and you'll be half-way to writing a story that's unputdownable.
Happy New Year and good luck.