Although Skyfall was a pretty awesome action movie, it did make the interesting decision to leave behind the story arc it had followed through Casino Royale and Quantum of Solace, and save that for Spectre; which might or might not be the last Daniel Craig Bond film? We're still not entirely sure.
Anyways, for this particular Bond film, James has received a posthumous message from the deceased M which sends him on an unauthorized mission in Mexico City. And *that* leads him on a path where he encounters the organization Spectre--which is apparently a big part of Bond lore in general. But I wouldn't know. I barely knew diddly-squat about James Bond before watching the Daniel Craig films. (Apparently that is where much of the criticism for this film came from--from longtime Bond watchers who saw everything coming. Sigh.)
But he comes into conflict with Spectre, who is led by a mysterious figure from Bond's past who has been manipulating recent events in Bond's life. In the meantime, Spectre and a bunch of politicians are working together to form a total surveillance program--their idea of world domination, and what is referred to as "George Orwell's worst nightmare."
Ultimately, Spectre turns out to be a fantastic action/adventure movie that manages to put itself just above Skyfall--carrying a multi-layered plot centering around a secret Illuminati-like organization (which I'm admittedly a sucker for), some brilliant action sequences (the opening sequence and the car chase being highlights), more strong acting from mostly the same cast we've seen before; brilliant cinematography, and a very satisfying ending.
Spectre received mixed reviews, but I feel like it completed its purpose quite well, being Sam Mendes' darker and grittier Bond film and one that can close the series on a high note and tie the loose ends together if this is indeed the last Bond film with Craig. When it's all said and done, Spectre definitely is a better movie than it gets credit for, and if you enjoyed the previous Bond films (Quantum of Solace not included), then it is definitely a must-watch.