When you’re really good at something, the time comes when the ghosts of your earlier accomplishments are your toughest competition. The English comedy team Simon Pegg and Nick Frost, who brilliantly satirized zombie movies in “Shaun of the Dead” and buddy-cop flicks in “Hot Fuzz,” gave audiences huge laughs, and gave themselves an extremely hard act to follow.

The pair’s third film, “Paul,” suffers by comparison. The film feels episodic, haphazard, more hit-and-miss onslaught than hilarious marksmanship. It pays homage to science fiction, manhunts, road movies, stoner comedies and fanboy culture. The in-jokes come fast and scattershot, citing “Battlestar Galactica,” “Aliens” and all things Spielberg. The Vasquez Rocks, featured in several “Star Trek” episodes, make a cameo appearance. But the film loses it with strained references to “Titanic,” “The Blues Brothers” and even the incest documentary “Capturing the Friedmans.”

The most crucial stumble, however, is structural. Rather than driving the story, Pegg and Frost become bystanders in a story about a runaway extraterrestrial.

Pegg and Frost play Graeme and Clive, aspiring comic-book creators with bad haircuts, limited social skills and a lot of T-shirts with comic-book motifs. They revel in the nerd nirvana that is Comic-Con, then take an RV on the open road on a tour of UFO sites throughout the Southwest.

The Englishmen feel a little alien in the United States, but things get seriously weird near Area 51. Following a close encounter with a friendly little green man in a stolen government sedan, the pair agree to assist his escape from the secret military compound where he’s been confined for 60 years.

Soon they’re on a life-and-death run from FBI Agent Zoil (Jason Bateman), angry rednecks and a vengeful daddy (John Carroll Lynch) whose daughter Ruth (Kristen Wiig) they have more or less kidnapped. The ensemble is not bad, but they consign Pegg and Frost to the sidelines when they should be front and center.

Missteps aside, there are lots of good ideas packed into the movie. Cool dude Paul (voiced, of course, by Seth Rogen ) is quite comfortable with human culture; he’s more down-to-earth than his British guardians. As he teaches them to shed their British reserve, relax and live in the moment, diffident Clive falls for Ruth, a sheltered fundamentalist Christian whose world is upended by her encounter with extraterrestrial life. When Paul zaps Ruth with a vision of the universe that refutes her dogma, she decides to break every commandment, but can’t get much beyond attempts to coin new swear words.

The culture clashes in this fish-out-of-water story are nicely handled, too. The Brits, who create comic books about three-breasted Martians, nevertheless consider themselves “men of science.” They regard the Biblical literalists they encounter much the way Paul sees humans in general: Odd primates. The script’s saber-sharp atheist humor puts a nervy jolt of social satire in the mix. Yet viewers who know what happened to the Bible’s Paul during his long-ago road trip will appreciate an added layer of irony in the film.

And there’s a sweetness of tone here that distinguishes “Paul” from the duo’s frenetically paced earlier work with director Edgar Wright. This time it’s American director Greg Mottola at the helm, delivering the emotional grace notes that made his hit “Superbad” and his criminally underseen followup “Adventureland” such warm experiences. “Paul” brims with affection for its characters, even as they become mere stick figures in a swirl of galloping, breakneck action.