Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a parody of the 1960 police drama TV show Dragnet about two no-nonsense Los Angeles Police Detectives. Dan Aykroyd plays the nephew of Joe Friday from the TV show. Tom Hanks plays his sidekick. The actual sidekick from the TV show, Harry Morgan, plays their boss. A terrorist organization that calls itself P.A.G.A.N. has been stealing and vandalizing around Los Angeles and the two detectives are sent to the Griffith Park zoo to investigate the thefts of some animals, including a missing giant "Central Bolivian Anaconda." A zookeeper shows them their "Boa Constrictor" (which is actually a Python) and tells them that the Anaconda eats whole live Groundhogs.
Later the detectives requisition some police department costumes that are supposed to make them look like local gang members and drive up into the San Gabriel Mountains to follow a lead. They discover a huge P.A.G.A.N. meeting where men are dressed in goat heads and fur leggings and dancing around chanting. A goat-headed man offers them some drugs from a large tray and Hanks fills his pocket with them. They blend in with the crowd and watch the leader of the Pagans bring out onto a balcony a woman in a wedding dress. He tells the crowd she's a virgin to be sacrificed to their god. She is then thrown into a pool of water below. Hanks and Aykroyd jump in to save her and discover the missing 30 foot long zoo Anaconda. The snake fights Hanks on one end and the woman and Aykroyd on the other. The Pagans watch the action on a big screen monitor above. The three try to fight off the Anaconda but they can't shake it until Hanks dumps a handful of pills down the snake's mouth. The Anaconda slows down, then passes out and somehow the three escape.
The giant Anaconda is a fake, of course, but it looks real enough for comedy in the foggy and poorly lit pool of water. The snake designer David B. Miller even gets a credit under special effects.