At the beginning of the film, it's mentioned in passing that "Aliens dropped these creatures down to Earth. They were bred specifically to kill humans" (when talking about the main antagonist monsters in the movie). Said aliens are never mentioned again. Way to just completely overlook what would have made a decent antagonist, or at least a decent backstory.
It early becomes obvious that the script makers for this movie never saw or read any harder sci-fi than Star Wars. The "Ursa" monsters are presented as a threat to humanity, and the reason why there are no humans on Earth any more. Except... they are big, bulky insect-ish creatures that fight only in close quarters (okay, they can spit venom too, but their range of action is still limited to less than twenty metres). Any man with a firearm of any fashion would easily defeat one from a comfortable distance. And to make matters worse, they have only one sense to find their prey with: they can smell fear, but are otherwise blind and deaf. One would wonder how these things manage to get around without bumping into trees or walking off cliffs. Either way, a Predator drone would easily clear a small country of those monsters in an afternoon, without any danger to the machine or its operator whatsoever. Even a Vietnam-era helicopter would hard counter any threat the Ursas would pose, only limited by ammunition and fuel.
Then comes the part of the movie that exists only to get Will Smith and his son to the (now-abandoned) Earth. They board this super-high-tech spaceship flying between two space colonies, but are surprised by a meteorite storm (Discovered when "It's two thousand kilometres to our starboard"). For some reason, the trained ship crew and its sensors can't detect the threat, but Will Smith's gut feeling can. Cue an immediate peltering of space rocks (seemingly hurled at them at highway speeds, which is hilariously slow for interstellar travel - unless they had been catching up to the cluster from the rear, in which case I can't suspend my disbelief any more - there's a limit to reasonable stupidity).
They decide to send the ship through a wormhole (why weren't they travelling by wormhole already, if that sends you wherever you want immediately?), and are spit out in high Earth orbit. Next follows a crash survived only by Smith and his son. The latter because he was strapped into his seat, the former because... no idea. The cockpit, where the two pilots were strapped into their seats, is shown intact. Still, the pilots and the rest of the crew apparently die (makes you wonder what kind of safety measures were implemented), while Will Smith, being thrown around the ship like a pea in a tennis ball, is all fine save for a pair of broken legs.
The emergency beacon in the bow part of the broken ship apparently couldn't survive a rough landing. Fine Fisher-Price quality of the ship's vital equipment. How realistic. Anyway, Smith jr. has to get to the rear part of the ship, which landed a hundred kilometres away, to pick up the spare emergency beacon and activate it, so people could find the wreckage and get the Smiths home. The Ursa the ship happened to be carrying also may have survived the crash, though that is presented as extremely unlikely and it would probably be badly hurt anyway. So, it's obvious it's alive and healthy and set to appear at the climax of the film.
Before young Smith leaves the ship, he is warned by his father: "This is Earth. Since the alien invasion, everything here has evolved specifically to kill humans". One would think specialization in human-killing would be a mighty disadvantage when the planet now is completely devoid of humans, but anyway. Cue a few scenes of spectacular wildlife completely ignoring Smith jr., until he directly attacks a monkey despite his father's direct orders and gets a whole flock after him. Said flock of super-monkeys, who are shown to outrun him in several consequent shots, but never catch up with him, all stop the chase when he jumps into a shallow river.
On his four-day trek towards the other half of the wreck, Smith jr. also has to be wary of two plot devices: Insane temperature differences between day and night, and lack of a breathable atmosphere. Luckily, there are so-called "hot spots" several places along the route (which he has to reach before sunset), and he has special capsules allowing him to breathe. A dose lasts for 24 hours, and of course he doesn't have enough to last all the way. But don't worry: he'll stumble upon the wreck just as the last dose is depleted. Why the dramatic temperature fluctuations don't lead to terrible storms, is never discussed. I uess the writers simply didn't care.
Then comes the definite facepalm moment of the movie: The beacon can't be turned on because there's no signal! A cloud of ions howering above the area blocks all coverage. First, a beacon transmits, it doesn't receive. It doesn't need signals, it's the source of the signals. Second, a cloud of ions would quickly regain neutral charge by means of electric discharge: A lihtning bolt, in other words (this is the reason why ions are never stable: they stop being ions after exchaning electrons with their surroundings). Note that back at the crashed ship, Will Smith can still even monitor the freakin' heart rate of his son. Those signals from his backpack apparently have no problems leaving the area.
Anyway, Smith jr. has to climb an eruptin volcano to get above the layer of ions. This involves a dive throuh aquifers in the volcano (which in real life would contain highly sulphurous, and probably boiling, water), a climb up throuh narrow cracks, and a fight with the Ursa at a narrow plateau hihg up in the mountainside. Smith jr. quickly learns to overcome his fears, becomes invisible to the monster, and slays it with a pair of knives. With all that volcanic ash in the air, the signal conditions probably wouldn't be any better up on the volcano, but at least the view is nice... After activating the beacon, he collapses from exhaustion, and presumably dies because the volcanic ash he inhaled turned to cement after mixing with moisture in his mouth/lungs (a very real threat near real volcanoes, and a key ingredient in cement is, indeed, volcanic ash). There is a happy ending, though, so I guess the writers didn't care about that either.
"The writers didn't care" seems to be the central theme of the movie. Now well. It provided some entertainment, at least.