This movie 'Head' is about the nature of free will, conceived and edited in a stream of consciousness style with no plot at all.
'Head' begins at the dedication of a bridge. As a local politician struggles with his microphone during the dedication speech, the "wacky, fun-loving" Monkees (Micky Dolenz, Davy Jones, Peter Tork, and Michael Nesmith), suddenly interrupt the ceremony by running through the assembled officials, to the sound of various horns and sirens....
The rest of the film shows what happened that led up to that.
Earlier... the four have just all kissed the same groupie, who tells them that they were indistinguishable. Throughout the film, they make their way both together and seperately through a series of unrelated vignettes, each being a different type of movie (a mystery, a war movie, a western, a desert adventure, etc...).
One memorable segment shows Micky trecking alone through a vast desert until he finds a soft drink vending machine standing on top of a sand dune. He is overjoyed to find that it is not a mirage, and happy that he has spare change, but infurated when the machine takes his money and gives him nothing back, for the machine is out of order. Then a small group of heavily armed soldiers, complete with a tank and all, arrive to fight Micky for control of the vending machine until Micky subdues the commanding officer, the entire unit surrenders to the lone Micky, who then takes advantage of this to commandeer the tank and blows up the broken down vending machine.
In each one segment, the Monkees try to deal with the fact that they're four real people in a real band that makes records for real people, but are also scripted characters in a fake TV band doing nothing except exactly what the director wants them to.
They continually try to prove to themselves that they're free and can make any choice they want. But no matter what they try deliberately flubbing their lines in scenes from their TV show, pointing out to other characters that they're really just actors making a movie, complaining to producers Jack Nicholson and Bob Rafelson... who are on the set but not part of the film, smashing through the painted paper walls, walking off the set and into the street, physically attacking other actors for no reason, and making everyone they encounter mad at them they discover that their every word and deed was predetermined to the finest detail by the script of the movie they're in and the director directing it.
For example, they forget their worries at a party where girls are go-go dancing. But a mirror on the wall reveals the movie camera shooting directly into it, recording the scene we are watching while Rafelson sits next to the camera in the mirror.
At one point, Peter actually discovers the answer to the free will contradiction in their reality. The four frequently find themselves inside a large black box from which they cannot escape. The box represents the constraints of being fictional characters unable to make any real choices. Peter announces that he will talk about the nature of conceptual reality, then informs the others that "it doesn't matter if we're in the box". He realizes that the difference between free will and pre-scripted action is illusory. As long as you can do anything you want, 'it doesn't matter' if your choices were known in advance by some powerful entity in a higher level of context outside the universe, because the situation in which you find yourself is identical to one in which there is no outer context and you really are free.
Unfortunately, the other three pay no attention to Pete's liberating revelation, which they characterize as navel-contemplating nonsense, and soon, even Pete forgets about it.
While being chased by everyone they've encountered (and disrupted) in the various vignettes, they run onto a bridge, shoving people out of the way. We see that they weren't being "wacky" at the beginning of the film; they were desperately trying to escape being mere scripted puppets. Finally, we see that they went to the bridge to make the ultimate assertion of free will. They jump off the edge and commit suicide, falling a very long way and slamming into the water far below.
However in the final scene, we see that this, too, was scripted. The film's director hauls their soaked bodies away in a huge aquarium while the four stare blankly through the glass, struggling under the water. Laughing, he rolls the aquarium into a slot at the studio warehouse, to be taken out when he wants to use them again in another movie.