Michael Fassbender stars opposite Michael Fassbender in “Alien: Covenant,” Ridley Scott’s latest prequel to his space-monster masterpiece, “Alien.” Having so perfectly played the diabolical android David in 2012’s “Prometheus,” Fassbender reprises the role and also plays Walter, a much more stable model. In keeping with tradition, “Alien: Covenant” focuses on a female astronaut who must outsmart a gooey-jawed predator, but it’s really Fassbender who plays both hero and villain here.

It’s an amusing twist, but not enough to keep this series from feeling a little stale. Some 40 years ago, “Alien” was supposedly pitched as “ ‘Jaws’ in space,” a reductive but crystal-clear description. Today it’s hard to say what the series has become. The layers of back story are multiplying and getting muddled — is the evil Weyland Corp. still a major player? — while the sets and special effects are growing increasingly elaborate. At the same time, “Alien: Covenant” still follows the familiar body-count structure of a slasher flick. The result is an uneven mix of serious science fiction, cheap thrills and, thanks to Fassbender’s dual role, a touch of camp.

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The film opens with a freshly minted David, bright eyed and barefoot, meeting his creator, Peter Weyland (Guy Pearce). As David sits at a piano and plays Wagner on command, it’s clear we’re watching a gifted child being spoiled by his parent. We won’t see David again until the crew of the colony ship Covenant finds him on a distant planet. Here, David rules as a kind of Dr. Moreau figure: isolated, insane, spouting Shelley’s “Ozymandias” and playing God with pets of his own creation.

The ship’s crew, by the way, is played by a fine cast: not just Katherine Waterston as our ostensible heroine, Daniels, but Billy Crudup as Oram, a self-doubting captain; Danny McBride as the cocksure pilot Tennessee; and Carmen Ejogo as the biologist Karine. Still, there’s no competing with Fassbender. He taunts himself, fights himself and, at one point, kisses himself. Not even the aliens can follow that act.

The movie’s sense of grandiosity is exactly what makes it feel a tad goofy. Despite its state-of-the-art effects and buckets of blood, “Alien: Covenant” seems likely to elicit almost as many giggles as screams.