American astronaut Frank Douglas was on a mission to identify satellites mysteriously not having been sent into orbit by any nation, but ground control lost contact with his space capsule (not shown).
Col. Steve Connors spearheads the search for Douglas's capsule after it is presumed to have parachuted back to Earth (not shown). Following up on a tip that a strange object had fallen to Earth near Chicago's Space Agency Astrophysical Laboratories, a helicopter patrol (unmarked) locates the nosecone of the capsule, but the pilot, Jim Taylor, mysteriously loses contact with Connors mid-sentence. Connors finds the capsule empty and the pilot far from the helicopter, dead and "horribly mangled in a way no one had ever seen before" (not shown).
Dr. Nora Kramer and Dr. Karl Schreiber visit Ruth Logan at home to update her that Frank Douglas is still missing. Ruth's concern is that, since her husband Harry Logan died, Frank had been like a father to their son (and Frank's nephew), Jimmy. In the middle of the meeting, Karl receives a phone call informing him that a body had been found. Ruth insists on accompanying Nora and Karl, only to learn on site that the body is Jim Taylor's, not Frank's. How Karl could know the capsule had landed but not know about the patrol pilot who had found it is left unexplained.
Dr. Frank Logan (brother of Dr. Tom Logan) shows Karl the corpse, which is described as "shriveled up like a dried prune" and "some kind of a burn" (not shown), in contrast to the body's prior description.
Back at SAAL, Dr. Frank Logan reveals to Col. Connors that Taylor died of radiation, and that the capsule couldn't have been the source. Logan concludes from this and the scorch marks that Schreiber dismissed as a prank that Douglas is still alive—a supposition Connors rejects.
Connors and Schreiber call in Dr. Chris Manning, the civilian head of the satellite investigation.
At an unrelated party, a jealous boyfriend drags off his girlfriend after she dances with another guy. He parks and they kiss to make up. She gets out of the car when he gets too aggressive. She turns around and witnesses Frank Douglas attack and kill her boyfriend (not shown). Manning matches the wounds to those of the helicopter pilot.
Dr. Frank Logan returns to the crash site with co-worker Henry Schwartz's Geiger counter to test his theory that a radioactive Frank Douglas killed the chopper pilot. He confirms his theory with his life.
Manning and Ruth Logan dine out, reminiscing over old times, but cut it short when they learn of Dr. Frank Logan's death. On site, Manning and Schreiber decide they must figure out why Frank Logan returned to the crash site to explain his death.
To that end, Connors introduces the military head of the project, Dr. Brent, to Dr. Nora Kramer, former assistant to Dr. Frank Logan and current assistant to Frank's brother Conrad. Kramer reveals Taylor's blood had fully desiccated. More significantly, a discrepancy in log books reveals that beginning six months ago, Douglas had received injections of anti-radiation treatment twice as effective as normal (Itinium 51 vs 50) and at twice the normal dosage. Results of overdose on pigs matched the characteristics of Monster Douglas—specifically, a lethal touch and doubling of size.
While synthesizing another batch of the antidote to Itinium 51 invented by Frank Logan, Kramer mentions there have been no monster attacks for three weeks. The narrator reveals this is because Dr. Conrad Logan has been keeping Monster Douglas in a storeroom, drugged with antidote and tranquilizer. Conrad is late in surreptitiously giving Douglas his evening injection due to Kramer's working late. While Conrad is absent to give Monster Douglas his injection, Monster Douglas surreptitiously escapes the storeroom, evades Conrad on the way to the lab, smashes the lab equipment, and steals all containers of the antidote (not shown), presumably for self-administration. The wreckage and theft force Conrad to 'fess up to Dr. Brent to his duplicity in hiding Monster Douglas (after three murders!)—to protect Douglas, so he claims. Dr. Brent must now 'fess up to Dr. Manning.
With the monster loose, attacks begin again. Monster Douglas returns to Ruth Logan's home, as the military had anticipated, but bullets do not stop him.
An odd scene seemingly unrelated to the plot is then interspliced. A truck driver rescues a flirtatious convertible driver out of gas. She repays him with a kiss. We later learn that Douglas boarded the back of the truck to get into Chicago (not shown)—the driver subsequently dying of radiation burns (also not shown).
Conrad decides Monster Douglas will soon run out of antidote, and so the public should be warned. Geiger counters become useless to spot Monster Douglas, as his radius of infection grows to be larger than the maximum distance of detection. Conrad and Connors bank on a blow dart with 3000 cc of antidote, to repeat the lethal shock to the system of returning to normal that an antidote overdose had had in animal studies.
Civil defense corners Monster Douglas in a Chicago sewer, but he suddenly disappears. Conrad simultaneously receives a telegram stating that Douglas is in fact alive and well, having been rescued 8000 miles from Chicago in a lifeboat in the North Atlantic with no memory of the prior weeks, suggesting either that Monster Douglas was an alien impersonating Douglas or Monster Douglas had turned into normal Douglas in a sort of time slip.
The film ends with a surprise revelation about the true fate of Frank Douglas and the monster's identity, upending prior assumptions.
Monster a Go-Go! concludes with a notorious twist that reframes the entire narrative, leaving viewers questioning the reality of the monster and the reliability of the investigation. The abrupt ending is infamous among cult film fans for its ambiguity and lack of resolution, cementing the film's status as a psychotronic oddity.
