The Boov are an alien race on the run from another alien race, the Gorg. Led by Captain Smek (Steve Martin), they choose Earth as a new home world - and relocate all the humans. But our view of the world after the alien invasion comes through "Oh" (Jim Parsons), a member of the Boov who is always making mistakes and is a social reject, and Tip (Rihanna), a human who wasn't relocated properly and is searching for her mother. It's typical buddy comedy: they meet, they hate each other, they're forced to work together, and they bond.
When you choose a formula that time-worn, you need to really, really do it well. And choosing Rihanna and Parsons as your two primaries isn't a good first step. Parsons has simply taken his "Sheldon" character from "Big Bang Theory" and cranked him up to 11 (the new "11" since arguably Sheldon was already at 11). Rihanna is at best a mediocre actress, and the jokes just aren't that great. For me, the presence of a soundtrack made almost entirely by Rihanna was a definite minus as well.
For all that, I somehow ended up kind of enjoying the film. It's got zero re-watch value, but it's colourful, inoffensive, and mildly amusing.